BSM4 : WORKSHOP PROCESSES
1. MILLING MACHINES
Introduction; classification and types; Size and specifications; Accessories attachment; Milling cutters;
Classification and types of milling cutter.; Nomenclature of cutter; Setup-operation ; Method of feeding work piece;
Operation on milling machine; Indexing (simple compound, differential angular) ; Helical milling cam milling ;
Cutting speed & ledge ; Machining time calculation; Milling operation compound with other operations
2. THE LATHE
Introduction, Functions, Types, Descriptions & Functions of Lathe Parts, Lathe Accessories & attachments, lathe
Operations.
3. GRINDING MACHINE
Introduction.; Types of Grading machines (Floor stand, Precision. Plain, cylindrical, universal centrals Internal,
surface disc); Special grinding machine, (Tool and cutter grinder, cam and and shape grinders); Shape of grinding
wheel; Grinding wheel designation as per- IS -551 -19-54; Grinding wheels
;
Grinding
wheel
elements
(abrasives - its types, Grain sizes, Grade, structure, bonding material etc.); Diamond wheel; Grinding wheel section;
Allowances for grinding wheel; Mounting of Grinding wheel; Dressing and cursing, of grinding wheel
4. BORING, BROACHING AND SAWING MACHINE
Introduction to Boring machines ; Types of Boring machine ; Boring haps and heads; Various operations using
boring heads; Boring operations using end supports; Introduction to Broaching machine ; Types of Broaching
machine; Broaching tool nomenclature; Types of Broaches; Broaching options compared with other process
(advantages & limitations.); External; Lubrication and cooling; Application of Broaching
5. GEAR MANUFACTURING
Gear tooth e1ement; Materials for Gears; Different methods of Gear manufacturing ; Gear generating methods; Gear
milling ; Gear shaping (Working principal of machine tool required Gear shaping cutters etc.) ; Gear Hibbing
(Working principal of machine tool required Gear hobbing operation) ; Gear finishing process ( Gear sharing
burnishing, grinding honing lapping
6. METAL FINISHING PROCESS
Introduction; Honing; Description and construction of honing tool.; Application of honing process; Lopping;
Description of Lapping compound and tool; Application of Lapping ; Super finishing process Burnishing - Polishing
- Buffing ; Application of super finishing operations.
7. PATTERN MAKING
Introduction, Pattern Materials, Pattern Making Tools, Pattern Allowances, Types of Patterns, Solid or Single Piece
Pattern, Split Pattern, Match Plate Pattern, Cope and Drag Pattern, Loose Piece Pattern, Gated Pattern, Sweep
Pattern, Skeleton Pattern, Shell Pattern, Segmental Pattern, Follow Board Pattern, Lagged-up Pattern, Left and Right
hand Pattern, Core Boxes, Colour coding for Pattern and Core Boxes.
8. MOULDING AND CORE MAKING
Introduction, Moulding Materials, Moulding Sand, Sand Binders, Sand Additives, Properties of Moulding Sand,
Classification of Moulding Sand, Grain Shape and Size of Sand, Preparation of Moulding Sand, Types of Moulding
Sand, Moulding Processes, Types of Moulds, Methods of Moulding, Methods of Green Sand Mould by Turn Over
Method, Gates and Risers, Types of Gates, Moulding Methods with Typical Patterns, Cores, Types of Cores, Core
Binders, Core Making, Core Setting, Core Shifting and Chaplets.
9. CASTING PROCESSES
Introduction, Permanent Mould Casting, Semi-permanent Mould Casting, Slush Casting, Die Casting, Centrifugal
Casting, Investment Casting, Shell Moulding Process, Continuous Casting, Defects in Casting, Cleaning of
Castings, Inspection of Castings, Design of Castings.
10. WELDING
Introduction, Weldability, Advantages and Disadvantages of Welded Joints, Types of Welded Joints, Cold Pressure
Welding, Types of Welded Joints, Fillet Welded Joints, Edge Preparation and Applications, Welding Positions,
Black Smith’s Forge Welding, Electric Resistance Welding, Types of Electric Resistance Welding, Spot Welding,
Roll Spot and Seam Welding, Projection Welding, Butt Welding, Percussion Welding, Arc Welding, Polarity in Arc
Welding, Comparison Between A.C. and D.C. Arc Welding, Types of Arc Welding, Electrodes for Arc Welding,
Arc Welding Equipment, Precautions in Arc Welding, Arc Welding Processes, Carbon Arc Welding, Metal Arc
Welding, Metallic Inert-gas (MIG)Arc Welding, Tungsten Inert-gas (TIG)Arc Welding, Atomic Hydrogen Welding,
Stud Welding, Submerged Arc Welding, Plasma Arc Welding, Flux Cored Arc Welding, Electro-slag Welding,
Electro-gas Welding, Thermit Welding, Solid State Welding, Modern Welding Processes, Basic Weld Symbols,
Supplementary Weld Symbols, Elements of a Welding Symbol, Standard Location of Elements of a Welding
Symbol, Gas Welding, Equipment for Oxy-acetylene Gas Welding, Welding Rods, Fluxes, Gas Flame, Gas Welding
Technique, Gas or Oxygen Cutting of Metals, Cutting Machines, Oxygen Lance Cutting, Arc Cutting, Oxygen Arc
Cutting Process, Welding of Various Metals, Testing of Welded Joints, Braze Welding, Soldering, Brazing.
11. RECENT DEVELOPMENT IN MANUFACTURING PROCESS
Introduction, Working of NC Machines tools, Classification of NC Machines, Programming for NC Machines,
Methods of Listing the Co-ordinates of points in NC System, Application of NC Machine, Advantages &
Disadvantages, Computer Numerical Control & Direct Numerical Control.
13. METAL CUTTING AND CUTTING TOOLS
Introduction, Types Of Cutting Tools, Measurement Of Forces, Types Of Chip, The Cutting Action Of Hand Tools,
Tool Life And Water, Machinability, Cutting Tool Materials, Cutting Fluid
14. DRILLING MACHINES
Introduction, Types Of Drilling Machines, Tools Holding Devices, Drilling Machine Operations, Types Of Drills,
Twist Drill Nomenclature, Drill Material, Reamer
15. SHAPER, PLANNER AND SLOTTING MACHINE
Introduction, principal parts, planner, planning machine parts, shaper vs. Planner, slotting machines, Slotting
machine parts, slotter operations, slotter tools
Unit 1
MILLING MACHINES
Structure
1.1.
Introduction
1.2.
Objectives
1.3.
Types of Milling Operations
1.4.
Cutting Conditions in Milling
1.5.
Milling Machines
1.6.
Machining Centers and Turning Centers
1.7.
Milling Machine Accessories and Attachments
1.8.
Summary
1.9.
Keywords
1.10.
Exercise
1.1.
Introduction
Milling is a machining operation in which a workpart is fed past a rotating cylindrical tool
with multiple cutting edges (in rare cases, a tool with one cutting edge, called a fly-cutter, is
used). The axis of rotation of the cutting tool is perpendicular to the direction of feed. This
orientation between the tool axis and the Feed direction is one of the features that distinguish
milling from drilling. In drilling, the cutting tool is fed in a direction parallel to its axis of
rotation. The cutting tool in milling is called a milling cutter and the cutting edges are called
teeth. The machine tool that traditionally performs this operation is a milling machine.
The geometric form created by milling is a plane surface. Other work geometries can be
created either by means of the cutter path or the cutter shape. Owing to the variety of shapes
possible and its high production rates, milling is one of the most versatile and widely used
machining operations. Milling is an interrupted cutting operation; the teeth of the milling
cutter enter and exit the work during each revolution. This interrupted cutting action subjects
the teeth to a cycle of impact force and thermal shock on every rotation. The tool material and
cutter geometry must be designed to withstand these conditions.
1.2.
Objectives
After studying this unit we are able to understand
− Types of Milling Operations
− Cutting Conditions in Milling
− Milling Machines
− Machining Centers and Turning Centers
1.3.
Types of Milling Operations
'There are two basic types of milling operations, shown in Figure 1: (a) peripheral milling
and (b) face milling.
Fig.1: Two basic types of milling operations:
(a) Peripheral or plain milling and (b) face milling
Peripheral Milling In peripheral milling, also called plain milling, the axis of the tool is
parallel to the surface being machined, and the operation is performed by cutting edges on the
outside periphery of the cutter. Several types of peripheral milling are shown in Figure 2:
(a) slab milling, the basic form of peripheral milling in which the cutter width extends
beyond the work piece on both sides;
(b) slotting, also called slot milling, in which the width of the cutter is less than the work
piece width, creating a slot in the work-when the cutter is very thin, this operation can be
used to mill narrow slots or cut a workpart in two, called saw milling;
(c) side milling, in which the cutter machines the side of the workpiece; and
(d) Straddle milling, the same as side milling, only cutting takes place on both sides of the
work. In peripheral milling, the rotation direction of the cutter distinguishes two forms of
milling: up milling and down milling, illustrated in Figure 3.
In up milling, also called conventional milling, the direction of motion of the cutter teeth is
opposite the feed direction when the teeth cut into the work. It is milling "against the feed."
In down milling, also called climb milling, the direction of cutter motion is the same as the
feed direction when the teeth cut the work. It is milling "with the feed." The relative
geometries of these two forms of milling result in differences in their cutting actions. In up
milling, the chip formed by each cutter tooth starts out very thin and increases in thickness
during the sweep of the cutter. In down milling, each chip starts out thick and reduces in
thickness throughout the cut. The length of a chip in down milling is less than in up milling
(the difference is exaggerated in our figure). This means that the cutter is engaged in the work
for less time per volume of material cut, and this tends to increase tool life in down milling.
The cutting force direction is tangential to the periphery of the cutter for the teeth that are
engaged in the work. In up milling, this has a tendency to lift the work part as the cutter teeth
exit the material. In down milling, this cutter force direction is downward, tending to hold the
work against the milling machine table.
Face Milling In face milling, the axis of the cutter is perpendicular to the surface being
milled, and machining is performed by cutting edges on both the end and outside periphery of
the cutter. As in peripheral milling, various forms of face milling exist, several of which are
shown in Figure 4:
(a) Conventional face milling, in which the diameter of the cutter is greater than the work
part width, so the cutter overhangs the work on both sides;
(b) Partial face milling, where the cutter overhangs the work on only one side;
(c) End milling, in which the cutter diameter is less than the work width, so a slot is cut into
the part;
(d) Profile milling, a form of end milling in which the outside periphery of a flat part is cut;
(e) Pocket milling, another Form of end milling used to mill shallow pockets into flat parts;
and
(f) Surface contouring, in which a ball-nose cutter (rather than square-end cutter) is fed back
and forth across the work along a curvilinear path at close intervals to create a threedimensional surface form. The same basic cutter control is required to machine the contours
of molds and dies, in which case the operation is called die sinking.
Fig. 2: Peripheral milling: (a) slab milling, (b) slotting, (c) side milling, and (d) straddle
milling.
Fig.3: two forms of milling with a 20-tooth cutter: (a) up milling, and (b) down milling.
Fig.4 Face milling: (a) conventional face milling, (b) partial face milling, (c) end milling,
(d) profile milling, (e) pocket milling, and (f) surface contouring.
1.4.
Cutting Conditions in Milling
The cutting speed is determined at the outside diameter of a milling cutter. This can be
converted to spindle rotation speed using a formula that should now be familiar:
1
The feed f in milling is usually given as a feed per cutter tooth; called the chip load, it
represents the size of the chip formed by each cutting edge. This can be converted to feed rate
by taking into account the spindle speed and the number of teeth on the cutter as follows:
2
Where fr = feed rate, mm/min (in/min); N = spindle speed, rev/min; n, =number of teeth on
the cutter; and f =chip load in mm/tooth (inltooth). Material removal rate in milling is
determined using the product of the cross sectional area of the cut and the feed rate.
Accordingly, if a slab-milling operation is cutting a work piece with width w at a depth d, the
material removal rate is
3
This neglects the initial entry of the cutter before full engagement. Eq. 3 can be applied to
end milling, side milling, face milling, and other milling operations, making the proper
adjustments in the computation of cross-sectional area of cut. The time required to mill a
workpiece of length L must account for the approach distance required to fully engage the
cutter. First, consider the case of slab milling, Figure 5. To determine the time to perform a
slab milling operation, the approach distance A to reach full cutter depth is given by
4
where d= depth of cut, mm (in); and D = diameter of the milling cutter, mm (in). The time to
mill the workpiece T,, is therefore
5
For face milling, it is customary to allow for the approach distance A plus an overtravel
distance 0. 'I'here are two possible cases as pictured in Figure 6. In both cases, A= 0. The first
case is when the cutter is centered over the rectangular workpiece. It is clear from Figure 6(a)
that A and 0 are each equal to half the cutter diameter.
Fig.5: Slab (peripheral) milling showing entry of cutter into the work piece
That is,
6
where D = cutter diameter, mm (in).
The second case is when the cutter is offset to one side of the work, as shown in Figure 6(b).
In this case, the approach and over travel distances are given by
7
where w = width of the cut, mm (in). Machining time in either case is therefore given by
8
Fig.6 Face milling showing approach and over travel distances for two cases: (a) when
cutter is centered over the work piece, and (b) when cutter is offset to one side over the
work.
1.5.
Milling Machines
Milling machines must provide a rotating spindle for the cutter and a table for fastening,
positioning, and feeding the workpart. Various machine tool designs satisfy these
requirements. To begin with, milling machines can be classified as horizontal or vertical. A
horizontal milling machine has a horizontal spindle, and this design is well suited for
performing peripheral milling (e.g., slab milling, slotting, side and straddle milling) on work
parts that are roughly cube shaped. A vertical milling machine has a vertical spindle, and
this orientation is appropriate for face milling, end milling, surface contouring, and die
sinking on relatively flat work parts. Other than spindle orientation, milling machines can be
classified into the following types:
(1)
knee-and-column,
(2)
bed type,
(3)
planer type,
(4)
tracer mills, and
(5)
CNC milling machines.
The knee-and-column milling machine is the basic machine tool for milling. It derives its
name from the fact that its two main components are a column that supports the spindle, and
a knee (roughly resembling a human knee) that supports the worktable. It is available as
either a horizontal or a vertical machine, as illustrated in Figure 7. In the horizontal version,
an arbor usually supports the cutter.
The arbor is basically a shaft that holds the milling cutter and is driven by the spindle. An
over arm is provided on horizontal machines to support the arbor. On vertical knee-andcolumn machines, milling cutters can be mounted directly in the spindle without an arbor.
One of the features of the knee-and-column milling machine that makes it so versatile is its
capability for worktable feed movement in any of the x-y-z axes. The worktable can be
moved in the x-direction, the saddle can be moved in the y-direction, and the knee can be
moved vertically to achieve the z-movement.
Two special knee-and-column machines should be identified. One is the universal milling
machine, Figure 8(a), which has a table that can be swiveled in a horizontal plane (about a
vertical axis) to any specified angle. This facilitates the cutting of angular shapes and helixes
on work parts. Another special machine is the ram mill, Figure 8(b), in which the tool head
containing the spindle is located on the end of a horizontal ram; the ram can be adjusted in
and out over the worktable to locate the cutter relative to the work. The tool head can also be
swiveled to achieve an angular orientation of the cutter with respect to the work. These
features provide considerable versatility in machining a variety of work shapes.
Fig.7: Two basic types of knee-and-column milling machine: (a) horizontal and (b)
vertical
Fig.8: Special types of knee-and-column milling machine: (a) universal-overarm, arbor,
and cutter omitted for clarity: and (b) ram type.
Bcd-type milling machines are designed for high production. They are constructed with
greater rigidity than knee-and-column machines, thus permitting them to achieve heavier feed
rates and depths of cut needed for high material removal rates. The characteristic construction
of the bed-type milling machine is shown in Figure 9. The worktable is mounted directly to
the bed of the machine tool, rather than using the less rigid knee-type design. This
construction limits the possible motion of the table to longitudinal feeding of the work past
the milling cutter. The cutter is mounted in a spindle head that can be adjusted vertically
along the machine column. Single spindle bed machines are called simplex mills, as in Figure
9, and are available in either horizontal or vertical models. Duplex mills use two spindle
heads. The heads are usually positioned horizontally on opposite sides of the bed to perform
simultaneous operations during one feeding pass of the work. Triplex mills add a third
spindle mounted vertically over the bed to further increase machining capability.
Fig.9: Simplex bed-type milling machine horizontal spindle
Planer type mills are the largest milling machines. Their general appearance and
construction are those of a large planer; the difference is that milling is performed instead of
planing. Accordingly, one or more milling heads are substituted for the single-point cutting
tools used on planers, and the motion of the work past the tool is a feed rate motion rather
than a cutting speed motion. Planer mills are built to machine very large parts. The worktable
and bed of the machine are heavy and relatively low to the ground, and the milling heads are
supported by a bridge structure that spans across the table.
A tracer mill, also called a profiling mill, is designed to reproduce an irregular part
geometry that has been created on a template. Using either manual feed by a human operator
or automatic feed by the machine tool, a tracing probe is controlled to follow the template
while a milling head duplicates the path taken by the probe to machine the desired shape.
Tracer mills can be divided into the following types: (1) x-y tracing, in which the template is
a flat shape with an outline to be profile milled using two-axis control; and (2) x-y-z tracing,
in which the probe follows a three dimensional pattern using three-axis control. Tracer mills
have been used for creating shapes that cannot easily be generated by a simple feeding action
of the workpart against the milling cutter. Their applications include the machining of molds
and dies. In recent years, many of the applications previously accomplished on tracer mills
have been taken over by computer numerical control (CNC) milling machines.
CNC milling machines are milling machines in which the cutter path is controlled by
numerical data rather than a physical template. They are especially suited to profile milling,
pocket milling, surface contouring, and die sinking operations, in which two or three axes of
the worktable must be simultaneously controlled to achieve the required cutter path. An
operator is normally required to change cutters as well as load and unload work parts.
1.6.
Machining Centers and Turning Centers
A machining center is a highly automated machine tool capable of performing multiple
machining operations under CNC control in one setup with minimal human attention. Typical
operations are those that use a rotating cutting tool, such as milling and drilling. The features
that make a machining center such a productive machine include:
Automatic tool changing. To change from one machining operation to the next, the
cutting tools must be changed. This is done on a machining center under NC program control
by an automatic tool-changer designed to exchange cutters between the machine tool spindle
and a tool storage drum Capacities of these drums commonly range from 16 to 80 cutting
tools.
Pallet shuttles. Some machining centers are equipped with two or more pallet
shuttles, which can be automatically transferred to the spindle to machine the workpart. With
two shuttles, the operator can be unloading the previous part and loading the next part while
the machine tool is engaged in machining the current part. This reduces nonproductive time
on the machine.
Automatic work part positioning: Many machining centers have more than three
axes. One of the additional axes is often designed as a rotary table to position the part at some
specified angle relative to the spindle. The rotary table permits the cutter to perform
machining on four sides of the part in a single setup. Machining centers are classified as
horizontal, vertical, or universal. The designation refers to spindle orientation. Horizontal
machining centers (HMCs) normally machine cube-shaped parts, in which the four vertical
sides of the cube can be accessed by the cutter. Vertical machining centers (VMCs) are suited
to flat parts on which the tool can machine the top surface. Universal machining centers have
work heads that swivel their spindle axes to any angle between horizontal and vertical.
Success of CNC machining centers led to the development of CNC turning centers. A
modern CNC turning center, is capable of performing various turning and related
operations, contour turning, and automatic tool indexing, all under computer control. In
addition, the most sophisticated turning centers can accomplish (I) work part gaging
(checking key dimensions after machining), (2) tool monitoring (sensors to indicate when the
tools are worn), (3) automatic tool changing when tools become worn, and even (4) automatic
work part changing at the completion of the work cycle.
Another type of machine tool related to machining centers and turning centers is the CNC
mill-turn center. This machine has the general configuration of a turning center; in addition, it
can position a cylindrical work part at a specified angle so that a rotating cutting tool (e.g.,
milling cutter) can machine features into the outside surface of the part, as illustrated in
Figure 10. An ordinary turning center does not have the capability to stop the work part at a
defined angular position, and it does not possess rotating tool spindles.
Further progress in machine tool technology has taken the mill-turn center one step further by
integrating additional capabilities into a single machine. The additional capabilities include
(1) combining milling, drilling, and turning with grinding, welding, and inspection
operations, all in one machine tool; (2) using multiple spindles simultaneously, either on a
single work piece or two different work pieces; and (3) automating the part handling function
by adding industrial robots to the machine. The terms multitasking machine and
multifunction machine are sometimes used for these products.
Fig.10: Operation of a mill-turn center: (a) example part with turned, milled, and
drilled surfaces; and (b) sequence of operations on a mill-turn center: (1) turn second
diameter, (2) mill flat with pan in programmed angular position, (3) drill hole with part
in same programmed position, and (4) cutoff.
Milling Cutters Classification of milling cutlers is closely associated with the milling
operations described in previous section. The major types of milling cutters are the following:
Plain milling cutters: These are used for peripheral or slab milling. As Figures 1(a) and 2(a)
indicate, they ate cylinder shaped with several rows of teeth. The cutting edges are usually
oriented at a helix angle (as in the figure) to reduce impact on entry into the work and these
cutters are called helical milling cutters. Tool geometry elements of a plain milling cutter
are shown in Figure 7.
Form milling cutters: These are peripheral milling cutters in which the cutting edges have a
special profile that is to be imparted to the work. An important application is in gear making,
in which the form milling cutter is shaped to cut the slots between adjacent gear teeth,
thereby leaving the geometry of the gear teeth.
Face milling cutters. These are designed with teeth that cut on both the side as well as the
periphery of the cutter. Face milling cutters can be made of HSS, as in Figure 1(b), or they
can be designed to use cemented carbide inserts. Figure 11 shows a four-tooth face-milling
cutter that uses inserts.
End milling cutters. As shown in Figure 4 (c), an end milling cutter looks like a drill bit, but
close inspection indicates that it is designed for primary cutting with its peripheral teeth
rather than its end. (A drill bit cuts only on its end as it penetrates into the work.) End mills
are designed with square ends, ends with radii, and ball ends. End mills can be used for face
milling, profile milling and pocketing, cutting slots, engraving, surface contouring, and die
sinking.
Fig: 12 Tool geometry elements of an 18-tooth plain milling cutter
Fig. 13: Tool geometry elements of a four-tooth face milling cutter: (a) side view and (b)
bottom view.
1.7.
Milling Machine Accessories and Attachments
a.
Arbors. Milling machine cutters can be mounted on several types of holding device.
The machinist must know the devices, and the purpose of each to make the most suitable
tooling setup for the operation to be performed. Technically, an arbor is a shaft on which a
cutter is mounted. For convenience, since there are so few types of cutter holders that are not
arbors, we will refer to all types of cutter holding devices as arbors. (1) Description. (a)
Milling machine arbors are made in various lengths and in standard diameters of 7/8, 1, 1 1/4,
and 1 1/2 inch. The shank is made to fit the tapered hole in the spindle, the other end is
threaded.
NOTE The threaded end may have left-handed or right-handed threads.
(b) Arbors are supplied with one of three tapers to fit the milling machine spindle (figure 4 on
the following page), the milling machines Standard taper, the Brown and Sharpe taper, and
the Brown and Sharpe taper with tang.
(c) The milling machine Standard taper is used on most machines of recent manufacture. It
was originated and designed by the milling machine manufacturers to make removal of the
arbor from the spindle much easier than will those of earlier design.
(d) The Brown and Sharpe taper is found mostly on older machines. Adapters or collets are
used to adapt these tapers to fit the machines whose spindles have milling machine Standard
tapers.
(e) The Brown and Sharpe taper with tang also are used on some of the older machines. The
tang engages a slot in the spindle to assist in driving the arbor.
(2) Standard Milling Machine Arbor
(a) The Standard milling machine arbor has a straight, cylindrical shape, with a Standard
milling taper on the driving end and a threaded portion on the opposite end to receive the
arbor nut. One or more milling cutters may be placed on the straight cylindrical shaft of the
arbor and held in position by means of sleeves and an arbor nut. The Standard milling
machine arbor is usually splined and has keys, used to lock each cutter to the arbor shaft.
Arbors are supplied in various lengths and standard diameters.
(b) The end of the arbor opposite the taper is supported by the arbor supports of the milling
machine. One or more supports are used, depending on the length of the arbor and the degree
of rigidity required. The end may be supported by a lathe center, bearing against the arbor nut
or by a bearing surface of the arbor fitting inside a bushing of the arbor support. Journal
bearings are placed over the arbor in place of sleeves where an intermediate arbor support is
positioned.
(c) The most common means of fastening the arbor in the milling machine spindle is by use
of a draw-in bolt (figure 14). The bolt threads into the taper shank of the arbor to draw the
taper into the spindle and hold it in place. Arbors secured in this manner are removed by
backing out the draw-in bolt and tapping the end of the bolt to loosen the taper.
Fig.14: Standard Milling Machine Arbor Installed
(3) Screw Arbor (figure 15 on the following page). Screw arbors are used to hold small
cutters that have threaded holes. These arbors have a taper next to the threaded portion to
provide alignment and support for tools that require a nut to hold them against a tapered
surface. A right-hand threaded arbor must be used for right-hand cutters; a left-hand threaded
arbor is used to mount left-hand cutters.
(4) Slitting Saw Milling Cutter Arbor (figure 15). The slitting saw milling cutter arbor is a
short arbor having two flanges between which the milling cutter is secured by tightening a
clamping nut. This arbor is used to hold the metal slitting saw milling cutters that are used for
slotting, slitting, and sawing operations.
(5) End Milling Cutter Arbor. The end milling cutter arbor has a bore in the end in which the
straight shank end milling cutters fit. The end milling cutters are locked in place by means of
a setscrew.
(6) Shell End Milling Cutter Arbor (figure 15). Shell end milling arbors are used to hold and
drive shell end milling cutters. The shell end milling cutter is fitted over the short boss on the
arbor shaft and is held against the face of the arbor by a bolt, or a retaining screw. The two
lugs on the arbor fit slots in the cutter to prevent the cutter from rotating on the arbor during
the machining operation. A special wrench is used to tighten and loosen a retaining
screw/bolt in the end of the arbor.
(7) Fly Cutter Arbor (figure 15). The fly cutter arbor is used to support a single-edge lathe,
shaper, or planer cutter bit, for boring and gear cutting operations on the milling machine.
These cutters, which can be ground to any desired shape, are held in the arbor by a locknut.
Fly cutter arbor shanks may have a Standard milling machine spindle taper, a Brown and
Sharpe taper, or a Morse taper.
Fig.15: Types of Milling Machine Arbors
b. Collets and Spindles.
(1) Description. Milling cutters that contain their own straight or tapered shanks are mounted
to the milling machine spindle with collets or spindle adapters which adapt the cutter shank to
the spindle.
(2) Collets. Collets for milling machines serve to step up or increase the taper sizes so that
small-shank tools can be fitted into large spindle recesses. They are similar to drilling
machine sockets and sleeves except that their tapers are not alike. Spring collets are used to
hold and drive straight-shanked tools. The spring collet chuck consists of a collet adapter,
spring collets, and a cup nut. Spring collets are similar to lathe collets. The cup forces the
collet into the mating taper, causing the collet to close on the straight shank of the tool.
Collets are available in several fractional sizes.
(3) Spindle Adapters. Spindle adapters are used to adapt arbors and milling cutters to the
standard tapers used for milling machine spindles. With the proper spindle adapters, any
tapered or straight shank cutter or arbor can be fitted to any milling machine, if the sizes and
tapers are standard.
c. Indexing Fixture.
(1) The indexing fixture is an indispensable accessory for the milling machine. Basically, it is
a device for mounting work pieces and rotating them a specified amount around the work
piece’s axis, as from one tooth space to another on a gear or cutter.
(2) The index fixture consists of an index head, also called a dividing head, and a footstock,
similar to the tailstock of a lathe. The index head and the footstock are attached to the
worktable of the milling machine by T slot bolts. An index plate containing graduations is
used to control the rotation of the index head spindle. The plate is fixed to the index head, and
an index crank, connected to the index head spindle by a worm gear and shaft, is moved
about the index plate. Work pieces are held between centers by the index head spindle and
footstock. Work pieces may also be held in a chuck mounted to the index head spindle, or
may be fitted directly into the taper spindle recess of some indexing fixtures.
Fig. 16: Indexing Fixture
(3) There are many variations of the indexing fixture. The name universal index head is
applied to an index head designed to permit power drive of the spindle so that helixes may be
cut on the milling machine. "Gear cutting attachment" is another name for an indexing
fixture; in this case, one primarily intended for cutting gears on the milling machine.
d. High-Speed Milling Attachment. The rate of spindle speed of the milling machine may be
increased from 1 1/2 to 6 times by the use of the high-speed milling attachment. This
attachment is essential when using cutters and twist drills which must be driven at a high rate
of speed in order to obtain an efficient surface speed. The attachment is clamped to the
column of the machine and is driven by a set of gears from the milling machine spindle.
e. Vertical Spindle Attachment. This attachment converts the horizontal spindle of a
horizontal milling machine to a vertical spindle. It is clamped to the column and driven from
the horizontal spindle. It incorporates provisions for setting the bead at any angle, from the
vertical to the horizontal, in a plane at right angles to the machine spindle. End milling and
face milling operations are more easily accomplished with this attachment, due to the fact
that the cutter and the surface being cut are in plain view.
f. Universal Milling Attachment. This device is similar to the vertical spindle attachment but
is more versatile. The cutter head can be swiveled to any angle in any plane, whereas the
vertical spindle attachment only rotates in one plane from the horizontal to the vertical.
g. Circular Milling Attachment. This attachment consists of a circular worktable containing
T-slots for mounting workpieces. The circular table revolves on a base attached to the milling
machine worktable. The attachment can be either hand or power driven, being connected to
the table drive shaft if power driven. It may be used for milling circles, arcs, segments, and
circular slots, as well as for slotting internal and external gears. The table of the attachment is
divided in degrees.
h. Offset Boring Head. The offset boring head is an attachment that fits to the milling
machine spindle and permits a single-edge cutting tool, such as a lathe cutter bit, to be
mounted off-center on the milling machine. Workpieces can be mounted in a vise attached to
the worktable and can be bored with this attachment.
4. Mounting and Indexing Work
a. General.
(1) An efficient and positive method of holding workpieces to the milling machine table is
essential if the machine tool is to be used to advantage. Regardless of the method used in
holding, there are certain factors that should be observed in every case. The workpiece must
not be sprung in clamping; it must be secured to prevent it from springing or moving away
from the cutter; and it must be so aligned that it may be correctly machined.
(2) Milling machine worktables are provided with several T-slots, used either for clamping
and locating the workpiece itself or for mounting various holding devices and attachments.
These T-slots extend the length of the table and are parallel to its line of travel. Most milling
machine attachments, such as vises and index fixtures, have keys or tongues on the underside
of their bases so that they may be located correctly in relation to the T-slots.
b. Methods of Mounting Workpieces.
(1) Clamping a Workpiece: To The Table. When clamping workpieces to the worktable of
the milling machine, the table and workpiece should be free from dirt and burrs. Workpieces
having smooth machined surfaces may be clamped directly to the table, provided the cutter
does not come in contact with the table surface during the machining operation. When
clamping workpieces with unfinished surfaces in this way, the table face should be protected
by pieces of soft metal. Clamps should be placed squarely across the workpiece to give a full
bearing surface. These clamps are held by Tslot bolts inserted in the T-slots of the table.
Clamping bolts should be placed as near to the workpiece as possible. When it is necessary to
place a clamp on an overhanging part of the workpiece, a support should be provided
between the overhang and the table, to prevent springing or possible breakage. A stop should
be placed at the end of the workpiece where it will receive the thrust of the cutter when heavy
cuts are being taken.
(2) Clasping a Workpiece to the Angle Plate. Workpieces clamped to the angle plate may be
machined with surfaces parallel, perpendicular, or at an angle to a given surface. When using
this method of holding a workpiece precautions should be taken, similar to those mentioned
in (1) above for clamping the workpiece-directly to the table. Angle plates may be of either
the adjustable or the nonadjustable type and are generally held in alignment by means of keys
or tongues that fit into the table T-slots.
(3) Clamping Workpieces in Fixtures. Fixtures are generally used in production work where a
number of identical pieces are to be machined. The design of the fixture is dependent upon
the shape of the piece and the operations to be performed. Fixtures are always constructed to
secure maximum clamping surfaces and are built to use a minimum number of clamps or
bolts, in order to reduce the time required for setting up the workpiece. Fixtures should
always be provided with keys to assure positive alignment with the table T-slots.
c. Indexing the Workpieces.
(1) General. Indexing equipment is used to hold the workpiece, and to provide a means of
turning it so that a number of accurately located speed cuts can be made, such as those
required in cutting tooth spaces on gears, milling grooves in reamers and taps, and forming
teeth on milling cutters.
The workpiece is held in a chuck, attached to a indexing head spindle, or mounted in between
a live center in the indexing head and dead center in the footstock. The center rest can be
used to support long slender work. The center of the footstock can be raised or lowered for
setting up tapered workpieces that require machining.
(2) Index Head. The bead of the indexing fixture contains an indexing mechanism, used to
control the rotation of the index head spindle in order to space or divide a workpiece
accurately. A simple indexing mechanism is illustrated in figure 10 on the following page. It
consists of a 40-tooth worm wheel fastened to the index head spindle, a single-cut worm, a
crank for turning the wormshaft, and an index plate and sector. Since there are 40 teeth in the
worm wheel, one turn of the index crank causes the worm wheel, and consequently the index
head spindle to, make one-fortieth of a turn; so 40 turns of the index crank revolves the
spindle one full turn.
(3) Plain Indexing. The following principles apply to basic indexing of workpieces:
(a) Suppose it is desired to mill a spur gear with 8 equally spaced teeth. Since, 40 turns of the
index crank will turn the spindle one full turn, one-eighth of 40, or 5 turns of the crank after
each cut, will space the gear for 8 teeth.
(b) The same principle applies whether or not the divisions required divide evenly into 40.
For example, if it is desired to index for 6 divisions, 6 divided into 40 equals 6 2/3 turns;
similarly, to index for 14 spaces, 14 divided into 40 equals 2 6/7 turns. Therefore, the
following rule can be derived: to determine the number of turns of the index crank needed to
obtain one division of any number of equal divisions on the workpiece, divide 40 by the
number of equal divisions desired (provided the worm wheel has 40 teeth, which is standard
practice).
Fig.17:. Simple Indexing Mechanism
(4) Index Plate. The index plate (figure 18 on the following page) is a round metal plate with
a series of six or more circles of equally spaced holes; the index pin on the crank can be
inserted in any hole in any circle. With the interchangeable plates regularly furnished with
most index heads,
the spacings necessary for most gears, boltheads, milling cutters, splines, and so forth, can be
obtained. The following sets of plates are standard equipment:
(a)
Brown and Sharpe type, 3 plates of 6 circles, each drilled as follows:
Plate 1- 15, 16, 17, 18, 19, 20 holes.
Plate 2- 21, 23, 27, 29, 31, 33 holes.
Plate 3- 37, 39, 41, 43, 47, 49 holes.
(b)
Cincinnati type, one plate drilled on both sides with circles divided as follows:
First side- 24, 25, 28, 30, 34, 37, 38, 39, 41, 42, 43 holes.
Second side- 46, 47, 49, 51,53, 54, 57, 58, 59, 62, 66 holes.
Fig. 18: Index Plate and Sector
(5) Indexing Operation. The two following examples show how the index plate is used to
obtain any desired part of a whole spindle turn by plain indexing.
(a) To Mill a Hexagon. Using the rule above, divide 40 by 6, which equals 6 2/3 turns, or six
full turns plus 2/3 of a turn on any circle whose number of holes is divisible by 3. Therefore,
six full turns of the crank plus 12 spaces on an 18-hole circle, or six full turns plus 26 spaces
on a 39-hole circle will produce the desired rotation of the workpiece.
(b) To Cut a Gear of 42 Teeth. Using the rule again, divide 40 by 42 which equals 40/42 or
20/21 turns, 40 spaces on a 42-hole circle or 20 spaces on a 21-hole circle. To use the rule
given, select a circle having a number of holes divisible by the required fraction of a turn
reduced to its lowest terms. The number of spaces between the holes gives the desired
fractional part of the whole
circle. When counting holes, start with the first hole ahead of the index pin.
(6)
Sector. The sector (figure 18 on the previous page) indicates the next hole in which
the pin is to be inserted and makes it unnecessary to count the holes when moving the index
crank after each cut. It consists of two radial, beveled arms which can be set at any angle to
each other and then moved together around the center of the index plate. Assume that it is
desired to make a series of cuts, moving the index crank 1 1/4 turns after each cut. Since the
circle has 20 turns, the crank must be turned one full turn plus 5 spaces after each cut. Set the
sector arms to include the desired fractional part of a turn, or 5 spaces, between the beveled
edges of its arms. If the first cut is taken with the index pin against the lefthand arm, to take
the next cut, move the pin once around the circle and into the hole against the right-hand arm
of the sector. Before taking the second cut, move the arms so that the left-hand arm is again
against the pin; this moves the right-hand arm another five spaces ahead of the pin. Then take
the second cut; repeat the operation until all the cuts have been completed.
NOTE
It is a good practice always to index clockwise on the plate.
(7)
Direct Indexing. The construction of some index heads permits the worm to be
disengaged from the worm wheel, making possible a quicker method of indexing, called
direct indexing. The index head is provided with a knob which, when turned through part of a
revolution, operates an eccentric and disengages the worm. Direct indexing is accomplished
by an additional index plate fastened to the index head spindle. A stationary plunger in the
index head fits the holes in the index plate. By moving the plate by hand to index directly, the
spindle and the workpiece rotate an equal distance. Direct index plates usually have 24 holes
and offer a quick means of milling squares, hexagons, taps, etc. Any number of divisions
which is a factor of 24 can be indexed quickly and conveniently by the direct indexing
method.
(8)
Differential Indexing. Sometimes a number of divisions are required which cannot be
obtained by simple indexing with the index plates regularly supplied. To obtain these
divisions a differential index head is used. The index crank is connected to the wormshaft by
a train of gears instead of by a direct coupling and with simple indexing. The selection of
these gears involves calculations similar to those used in calculating change gear ratio for
cutting threads on a lathe.
1.8.
Summary
In this unit we have studied
− Types of Milling Operations
− Cutting Conditions in Milling
− Milling Machines
− Machining Centers and Turning Centers
1.9.
Keywords
Peripheral Milling
Face Milling
Peripheral milling
slab milling
slotting
side milling
straddle milling
Milling Cutters
Arbors
Collets and Spindles
1.10.
Exercise
1. Explain the different types of milling machines.
2. Explain the accessories and attachment of milling machines
3. Classify the milling cutter.
4. Explain the methods of feeding work piece
Unit 2
THE LATHE
Structure
2.1.
Introduction
2.2.
Objectives
2.3.
Accessories and Attachments
2.4.
Other Lathes and Turning Machines
2.5.
Operations of Lathe
2.6.
Summary
2.7.
Keywords
2.8.
Exercise
2.1.
Introduction
The basic lathe used for turning and related operations is an engine lathe. It is a versatile
machine tool, manually operated, and widely used in low and medium production. The term
engine dates from the time when these machines were driven by steam engines.
Fig.1: Lathe
Engine Lathe Technology Figure 1 is a sketch of an engine lathe showing its principal
components. The headstock contains the drive unit to rotate the spindle, which rotates the work.
Opposite the headstock is the tailstock, in which a center is mounted to support other end of the
work piece.
The cutting tool is held in a tool post fastened to the cross-slide, which is assembled to the
carriage.
The carriage is designed to slide along the ways of the lathe in order to feed the tool parallel to
the axis of rotation. The ways are like tracks along which the carriage rides, and they are made
with great precision to achieve a high degree of parallelism relative to the spindle axis.
The ways are built into the bed of the lathe, providing a rigid frame for the machine tool. The
carriage is driven by a lead screw that rotates at the proper speed to obtain the desired feed rate.
The cross-slide is designed to feed in a direction perpendicular to the carriage movement. Thus,
by moving the carriage, the tool can be fed parallel to the work axis to perform straight turning;
or by moving the cross-slide, the tool can be fed radially into the work to perform facing, form
turning, or cutoff operations.
The conventional engine lathe and most other machines described in this section are horizontal
turning machines; that is, the spindle axis is horizontal. This is appropriate for the majority of
turning jobs, in which the length is greater than the diameter.
For jobs in which the diameter is large relative to length and the work is heavy, it is more
convenient to orient the work so that it rotates about a vertical axis; these are vertical turning
machines. The size of a lathe is designated by swing and maximum distance between centers.
The swing is the maximum work part diameter that can be rotated in the spindle, determined as
twice the distance between the centerline of the spindle and the ways of the machine. The actual
maximum size of a cylindrical work piece that can be accommodated on the lathe is smaller than
the swing because the carriage and cross-slide assembly are in the way. The maximum distance
between centers indicates the maximum length of a work piece that can be mounted between
headstock and tailstock centers. For example, a 350 mm x 1.2 m (14 in x 48 in) lathe designates
that the swing is 350 mm (14 in) and the maximum distance between centers is 1.2 m (48 in).
2.2.
Objectives
After studying this unit we are able to understand
− Accessories and Attachments
− Other Lathes and Turning Machines
− Operations of Lathe
2.3.
Accessories and Attachments
There are four common methods used to hold work parts in turning. These work holding
methods consist of various mechanisms to grasp the work, center and support it in position along
the spindle axis, and rotate it. The methods, illustrated in Figure 2, are (a) mounting the work
between centers, (b) chuck, (c) collet, and (d) face plate. Our video clip on work holding
illustrates the various aspects of fixturing for turning and other machining operations.
Fig. 2
Holding the work between centers refers to the use of two centers, one in the headstock and the
other in the tailstock, as in Figure 2(a). This method is appropriate for parts with large length-todiameter ratios. At the headstock center, a device called a dog is attached to the outside of the
work and is used to drive the rotation from the spindle. The tailstock center has a cone-shaped
point which is inserted into a tapered hole in the end of the work. The tailstock center is either a
"live" center or a "dead" center.
A live center rotates in a bearing in the tailstock, so that there is no relative rotation between the
work and the live center, hence, no friction. In contrast, a dead center is fixed to the tailstock, so
that it does not rotate; instead, the work piece rotates about it. Because of friction, and the heat
buildup that results, this setup is normally used at lower rotational speeds. The live center can be
used at higher speeds.
The chuck, Figure 2(b) is available in several designs, with three or four jaw to grasp the
cylindrical work part on its outside diameter. The jaws are often designed so they can also grasp
the inside diameter of a tubular part. A self-centering chuck has a mechanism to move the jaws
in or out simultaneously, thus centering the work at the spindle axis. Other chucks allow
independent operation of each jaw. Chucks can be used with or without a tailstock center. For
parts with low length-to-diameter ratios, holding the part in the chuck in a cantilever fashion is
usually sufficient to withstand the cutting forces. For long work bars, the tailstock center is
needed for support.
A collet consists of a tubular bushing with longitudinal slits running over half its length and
equally spaced around its circumference, as in Figure 2 (c). The inside diameter of the collet is
used to hold cylindrical work such as bar stock. Owing to the slits, one end of the collet can be
squeezed to reduce its diameter and provide a secure grasping pressure against the work.
Because there is a limit to the reduction obtainable in a collet of any given diameter, these work
holding devices must be made in various sizes to match the particular work part size in the
operation.
A faceplate, Figure 2(d), is a work holding device that fastens to the lathe spindle and is used to
grasp parts with irregular shapes. Because of their irregular shape, these parts cannot be held by
other work holding methods. The faceplate is therefore equipped with the custom-designed
clamps for the particular geometry of the part.
2.4.
Other Lathes and Turning Machines
In addition to the engine lathe, other turning machines have been developed to satisfy particular
functions or to automate the turning process. Among these machines are
(1) Tool room lathe,
(2) Speed lathe,
(3) Turret lathe,
(4) Chucking machine,
(5) Automatic screw machine, and
(6) Numerically controlled lathe.
The tool room lathe and speed lathe are closely related to the engine lathe. The tool room lathe
is smaller and has a wider available range of speeds and feeds. It is also built for higher accuracy,
consistent with its purpose of fabricating components for tools, fixtures, and other high-precision
devices.
The speed lathe is simpler in construction than the engine lathe. It has no carriage and crossslide assembly, and therefore no lead screw to drive the carriage. The cutting tool is held by the
operator using a rest attached to the lathe for support. The speeds are higher on a speed lathe, but
the number of speed settings is limited. Applications of this machine type include wood turning,
metal spinning, and polishing operations.
A turret lathe is a manually operated lathe in which the tailstock is replaced by a turret that
holds up to six cutting tools. These tools can be rapidly brought into action against the work one
by one by indexing the turret. In addition, the conventional tool post used on an engine lathe is
replaced by a four-sided turret that is capable of indexing up to four tools into position. Hence,
because of the capacity to quickly change from one cutting tool to the next, the turret lathe is
used for high-production work that requires a sequence of cuts to be made on the part.
As the name suggests, a chucking machine (nicknamed chucker) uses a chuck in its spindle to
hold the work part. The tailstock is absent on a chucker, so parts cannot be mounted between
centers. This restricts the use of a chucking machine to short, lightweight parts. The setup and
operation are similar to a turret lathe except that the feeding actions of the cutting tools are
controlled automatically rather than by a human operator. The function of the operator is to load
and unload the parts.
A bar machine is similar to a chucking machine except that a collet is used (instead of a chuck),
which permits long bar stock to be fed through the headstock into position. At the end of each
machining cycle, a cutoff operation separates the new part. The bar stock is then indexed forward
to present stock for the next part. Feeding the stock as well as indexing and feeding the cutting
tools are accomplished automatically. Owing to its high level of automatic operation, it is often
called an automatic bar machine. One of its important applications is in the production of screws
and similar small hardware items; the name automatic screw machine is frequently used for
machines used in these applications.
Bar machines can be classified as single spindle or multiple spindles.
A single spindle bar machine has one spindle that normally allows only one cutting tool to be
used at a time on the single work part being machined. Thus, while each tool is cutting the work,
the other tools are idle. (Turret lathes and chucking machines are also limited by this sequential,
rather than simultaneous, tool operation). To increase cutting tool utilization and production rate,
multiple spindle bar machines are available. These machines have more than one spindle, so
multiple parts are machined simultaneously by multiple tools.
For example, a six-spindle automatic bar machine works on six parts at a time, as in Figure 2.3.
At the end of each machining cycle, the spindles (including collets and work bars) are indexed
(rotated) to the next position. In our illustration, each part is cut sequentially by five sets of
cutting tools, which takes six cycles (position 1 is for advancing the bar stock to a "stop"). With
this arrangement, a part is completed at the end of each cycle. As a result, a six-spindle automatic
screw machine has a very high production rate.
Fig.3
The sequencing and actuation or the motions on screw machines and chucking machines have
traditionally been controlled by cams and other mechanical devices. The modern form of control
is computer numerical control (CNC), in which the machine tool operations are controlled by a
"program of instructions". CNC provides a more sophisticated and versatile means of control
than mechanical devices. CNC has led to the development of machine tools capable of more
complex machining cycles and part geometries, and a higher level of automated operation than
conventional screw machines and chucking machines. The CNC lathe is an example of these
machines in turning. It is especially useful for contour turning operations and close tolerance
work. Today, automatic chuckers and bar machines are implemented by CNC.
2.5.
Operations of Lathe
A variety of other machining operations can be performed on a lathe in addition to turning; these
include the following, illustrated in Figure 4:
(a) Facing. The tool is fed radially into the rotating work on one end to create a flat surface
on the end.
(b) Taper turning. Instead of feeding the tool parallel to the axis of rotation of the work, the
tool is fed at an angle, thus creating a tapered cylinder or conical shape.
(c) Contour turning. Instead of feeding the tool along a straight line parallel to the axis of
rotation as in turning, the tool follows a contour that is other than straight, thus creating a
contoured form in the turned part.
(d) Form turning. In this operation, sometimes called forming, the tool has a shape that is
imparted to the work by plunging the tool radially into the work.
(e) Chamfering. The cutting edge ol the tool is used to cut an angle on the corner of the
cylinder, forming what is called a "chamfer."
(f) Cutoff: The tool is fed radially into the rotating work at some location along its length to
cut off the end of the part. This operation is sometimes referred to as parting.
(g) Threading. A pointed tool is fed linearly across the outside surface of the rotating work
part in a direction parallel to the axis of rotation at a large effective feed rate, thus
creating threads in the cylinder.
(h) Boring. A single-point tool is fed linearly, parallel to the axis of rotation, on the inside
diameter of an existing hole in the part.
(i) Drilling. Drilling can be performed on a lathe by feeding the drill into the rotating work
along its axis. Reaming can be performed in a similar way.
(j) Knurling. This is not a machining operation because it does not involve cutting of
material. Instead, it is a metal forming operation used to produce a regular crosshatched
pattern in the work surface.
Fig. 4: Machining operations other than turning that are performed on a lathe: (a) facing,
(b) taper turning, (c) contour turning, (d) form turning, (e) chamfering, (0 cutoff, (g)
threading, (h) boring, (i) drilling, and (j) knurling.
Most lathe operations use single-point tools. Turning, facing, taper turning, contour turning,
chamfering, and boring are all performed with single-point tools. A threading operation is
accomplished using a single-point tool designed with a geometry that shapes the thread. Certain
operations require tools other than single-point. Form turning is performed with a specially
designed tool called a form tool. The profile shape ground into the tool establishes the shape of
the work part. A cutoff tool is basically a form tool. Drilling is accomplished by a drill bit.
Knurling is performed by a knurling tool, consisting of two hardened forming rolls, each
mounted between centers. The forming rolls have the desired knurling pattern on their surfaces.
To perform knurling, the tool is pressed against the rotating work part with sufficient pressure to
impress the pattern onto the work surface.
2.6.
Summary
In this unit we have studied
− Accessories and Attachments
− Other Lathes and Turning Machines
− Operations of Lathe
2.7.
Keywords
Facing
Taper turning
Contour turning
Form turning
Chamfering
Cutoff
Threading
Boring
Drilling
Reaming
Knurling
Collet
faceplate
Tool room lathe
Speed lathe
Turret lathe
Chucking machine
Automatic screw machine
Numerically controlled lathe
2.8.
Exercise
1. Explain the accessories and attachments of lathe.
2. Explain the other lathes and turning machines
3. How to operate the lathe?
Unit 3
Grinding Machine
Structure
3.1.
Introduction
3.2.
Objectives
3.3.
Grinding Operations and Grinding Machines
3.3.1. Surface Grinding
3.3.2. Cylindrical Grinding
3.3.3. Centerless Grinding
3.3.4. Creep Feed Grinding
3.3.5. Other Grinding Operations
3.4.
Grinding Wheel Elements
3.4.1. Abrasive Material
3.4.2. Grain Size
3.4.3. Diamond
3.4.4. Grades
3.4.5. Bonding Materials
3.5.
Grinding Wheel Selection
3.6.
Grinding Allowance
3.7.
Mounting of Grinding Wheels
3.8.
Dressing, Truing and Balancing
3.9.
Summary
3.10.
Keywords
3.11.
Exercise
3.1.
Introduction
Abrasive machining involves material removal by the action of hard, abrasive particles that are
usually in the form of a bonded wheel. Grinding is the most important of the abrasive processes.
In terms of number of machine tools in use, grinding is the most common of ail metalworking
operations. Other abrasive processes include honing, lapping, super finishing, polishing, and
buffing. The abrasive machining processes are generally used as finishing operations, although
some abrasive processes are capable of high material removal rates rivaling those of
conventional machining operations.
Grinding is a material removal process accomplished by abrasive particles that are contained in a
bonded grinding wheel rotating at very high surface speeds. The grinding wheel is usually diskshaped, and is precisely balanced for high rotational speeds. The reader can see grinding in
action in our video clip titled Basics of Grinding.
Grinding can be likened to the milling process. Cutting occurs on either the periphery or the face
of the grinding wheel, similar to peripheral milling and face milling. Peripheral grinding is much
more common than face grinding. The rotating grinding wheel consists of many cutting teeth
(the abrasive particles), and the work is fed relative to the wheel to accomplish material removal.
Despite these similarities, there are significant differences between grinding and milling: (1) the
abrasive grains in the wheel are much smaller and more numerous than the teeth on a milling
cutter; (2) cutting speeds in grinding are much higher than in milling; (3) the abrasive grits in a
grinding wheel are randomly oriented and possess on average a very high negative rake angle;
and (4) a grinding wheel is self-sharpening-as the wheel wears, the abrasive particles become
dull and either fracture to create fresh cutting edges or are pulled out of the surface of the wheel
to expose new grains.
3.2.
Objectives
After studying this unit we are able to understand
− Grinding Operations and Grinding Machines
− Surface Grinding
− Cylindrical Grinding
− Centerless Grinding
− Creep Feed Grinding
− Other Grinding Operations
− Grinding Wheel Elements
− Abrasive Material
− Grain Size
− Diamond
− Grades
− Bonding Materials
− Grinding Wheel Selection
− Grinding Allowance
− Mounting of Grinding Wheels
− Dressing, Truing and Balancing
3.3.
Grinding Operations and Grinding Machines
Grinding is traditionally used to finish parts whose geometries have already been created by
other operations. Accordingly, grinding machines have been developed to grind plain flat
surfaces, external and internal cylinders, and contour shapes such as threads. The contour shapes
are often created by special formed wheels that have the opposite of the desired contour to be
imparted to the work. Grinding is also used in tool rooms to form the geometries on cutting tools.
In addition to these traditional uses, applications of grinding are expanding to include more high
speed, high material removal operations.
Our discussion of operations and machines in this section includes the following types: (1)
surface grinding, (2) cylindrical grinding, (3) center less grinding, (4) creep feed grinding, and
(5) other grinding operations.
3.3.1.
Surface Grinding
Surface grinding is normally used to grind plain flat surfaces. It is performed using either the
periphery of the grinding wheel or the fiat face of the wheel. Since the work is normally held in a
horizontal orientation, peripheral grinding is performed by rotating the wheel about a horizontal
axis, and face grinding is performed by rotating the wheel about a vertical axis. In either case, the
relative motion of the workpart is achieved by reciprocating the work past the wheel or by
rotating it. These possible combinations of wheel orientations and workpart motions provide the
four types of surface grinding machines illustrated in Figure 1. Of the four types, the horizontal
spindle machine with reciprocating worktable is the most common, shown in Figure 2. Grinding
is accomplished by reciprocating the work longitudinally under the wheel at a very small depth
(in feed) and by feeding the wheel transversely into the work a certain distance between strokes.
In these operations, the width of the wheel is usually less than that of the workpiece. In addition
to its conventional application, a grinding machine with horizontal spindle and reciprocating
table can be used to form special contoured surfaces by employing a formed grinding wheel.
Instead of feeding the wheel transversely across the work as it reciprocates, the wheel is plungefed vertically into the work. The shape of the formed wheel is therefore imparted to the work
surface. Grinding machines with vertical spindles and reciprocating tables are set up so that the
wheel diameter is greater than the work width. Accordingly, these operations can be performed
without using a transverse feed motion. Instead, grinding is accomplished by reciprocating the
work past the wheel, and feeding the wheel vertically into the work to the desired dimension.
This configuration is capable of achieving a very flat surface on the work. Of the two types of
rotary table grinding in Figure 1(b) and (d), the vertical spindle machines are more common.
Owing to the relatively large surface contact area between wheel and workpart, vertical spindlerotary table grinding machines are capable of high metal removal rates when equipped with
appropriate grinding wheels.
Figure 1: Four types of surface grinding: (a) horizontal spindle with reciprocating worktable, (b)
horizontal spindle with rotating worktable, (c) vertical spindle with reciprocating worktable, and
(d) vertical spindle with rotating worktable.
Figure 2: Surface grinders with horizontal Bed spindle and reciprocating worktable.
3.3.2.
Cylindrical Grinding
As its name suggests, cylindrical grinding is used for rotational parts. These grinding operations
divide into two basic types, Figure 3: (a) external cylindrical grinding and (b) internal
cylindrical grinding. External cylindrical grinding(also called center-type grinding to
distinguish it from center less grinding) is performed much like a turning operation. The grinding
machines used for these operations closely resemble a lathe in which the tool post has been
replaced by a high-speed motor to rotate the grinding wheel. The cylindrical work piece is
rotated between centers to provide a surface speed of 18-30 d min (60-100 ft/min) 1161, and the
grinding wheel, rotating at 1200-2000 mlmin (4000-6500 ft/min), is engaged to perform the cut.
There are two types of feed motion possible, traverse feed and plunge-cut, shown in Figure 4. In
traverse feed, the grinding wheel is fed in a direction parallel to the axis of rotation of the work
part. The in feed is set within a range typically from 0.0075 to 0.075 mm (0.0003-0.003 in). A
longitudinal reciprocating motion is sometimes given to either the work or the wheel to improve
surface finish. In plunge-cut, the grinding wheel is fed radially into the work. Formed grinding
wheels use this type of feed motion.
Figure 3: Two types of cylindrical grinding: (a) external, and (b) internal.
External cylindrical grinding is used to finish parts that have been machined to approximate size
and heat treated to desired hardness. Parts include axles, crankshafts, spindles, bearings and
bushings, and rolls for rolling mills. The grinding operation produces the final size and required
surface finish on these hardened parts.
Internal cylindrical grinding operates somewhat like a boring operation. The work piece is
usually held in a chuck and rotated to provide surface speeds of 20-60 m/min (75-200 ft/min) .
Wheel surface speeds similar to external cylindrical grinding are used. The wheel is fed in either
of two ways: traverse feed, Figure 3 (b), or plunge feed. Obviously, the wheel diameter in
internal cylindrical grinding must be smaller than the original bore hole. This often means that
the wheel diameter is quite small, necessitating very high rotational speeds in order to achieve
the desired surface speed. Internal cylindrical grinding is used to finish the hardened inside
surfaces of bearing races arid bushing surfaces.
3.3.3.
Centerless Grinding
Centerless grinding is an alternative process for grinding external and internal cylindrical
surfaces. As its name suggests, the work piece is not held between centers. This results in a
reduction in work handling time; hence, centerless grinding is often used for high-production
work. The setup for external center less grinding (Figure 5), consists of two wheels: the
grinding wheel and a regulating wheel. The work parts, which may be many individual short
pieces or long rods (e.g.,3-4m long), are supported by a rest blade and fed through between the
two wheels. The grinding wheel does the cutting, rotating at surface speeds of 1200-1800 m/min
(4000-6000 ft/min). The regulating wheel rotates at much lower speeds and is inclined at a slight
angle I to control through feed of the work. The following equation can be used to predict
through feed rate, based on inclination angle and other parameters of the process:
ft =
Dr Nr sin I
where f, = through feed rate, mm/min (in/min); D,. = diameter of the regulating wheel, mm (in);
Nr = rotational speed of the regulating wheel, rev/min; and I = inclination angle of the regulating
wheel.
The typical setup in internal centerless grinding is shown in Figure 6. In place of the rest blade,
two support rolls are used to maintain the position of the work. The regulating wheel is tilted at a
small inclination angle to control the feed of the work past the grinding wheel. Because of the
need to support the grinding wheel, through feed of the work as in external center less grinding is
not possible. Therefore this grinding operation cannot achieve the same high-production rates as
in the external center less process. Its advantage is that it is capable of providing very close
concentricity between internal and external diameters on a tubular part such as a roller bearing
race.
Figure 4: two types of feed motion in external cylindrical grinding: (a) traverse feed, and (b)
plunge-cut.
Figure 5: External centerless grinding.
3.3.4.
Creep Feed Grinding
A relatively new form of grinding is creep feed grinding, developed around 1958. Creep feed
grinding is performed at very high depths of cut and very low reed rates; hence, the name creep
feed. The comparison with conventional surface grinding is illustrated in Figure 6.
Depths of cut in creep feed grinding are 1000 to 10,000 times greater than in conventional
surface grinding, and the feed rates are reduced by about the same proportion. However, material
removal rate and productivity are increased in creep feed grinding because the wheel is
continuously cutting. This contrasts with conventional surface grinding in which the
reciprocating motion of the work results in significant lost time during each stroke. Creep feed
grinding can be applied in both surface grinding and external cylindrical grinding. Surface
grinding applications include grinding of slots and profiles. The process seems especially suited
to those cases in which depth-to-width ratios are relatively large. The cylindrical applications
include threads, formed gear shapes, and other cylindrical components. The term deep grinding
is used in Europe to describe this external cylindrical creep feed grinding applications.
The introduction of grinding machines designed with special features for creep feed grinding has
spurred interest in the process. The features include high static and dynamic stability, highly
accurate slides with reduced tendency to stick-slip, increased spindle power (two to three times
the power of conventional grinding machines), consistent table speeds for low feeds, highpressure grinding fluid delivery systems, and dressing systems capable of dressing the grinding
wheels during the process. Typical advantages of creep feed grinding include (1) high material
removal rates, (2) improved accuracy for formed surfaces, and (3) reduced temperatures at the
work surface.
Figure 6: Comparison of (a) conventional surface grinding and (b) creep feed grinding
3.3.5.
Other Grinding Operations
Several other grinding operations should be briefly mentioned to complete our review. These
include tool grinding, jig grinding, disc grinding, snag grinding, and abrasive belt grinding.
Cutting tools are made of hardened tool steel and other hard materials. Tool grinders are special
grinding machines of various designs to sharpen and recondition cutting tools. They have devices
for positioning and orienting the tools to grind the desired surfaces at specified angles and radii.
Some tool grinders are general purpose while others cut the unique geometries of specific tool
types. General-purpose tool and cutter grinders use special attachments and adjustments to
accommodate a variety of tool geometries. Single-purpose tool grinders include gear cutter
sharpeners, milling cutter grinders of various types, broach sharpeners, and drill point grinders.
Jig grinders are grinding machines traditionally used to grind holes in hardened steel parts to
high accuracies. The original applications included press working dies and tools. Although these
applications are still important, jig grinders are used today in a broader range of applications
where high accuracy and good finish are required on hardened components. Numerical control is
available on modern jig grinding machines to achieve automated operation.
Disc grinders are grinding machines with large abrasive discs mounted on either end of a
horizontal spindle as in Figure 7. The work is held (usually manually) against the flat surface of
the wheel to accomplish the grinding operation. Some disc grinding machines have double
opposing spindles. By setting the discs at the desired separation, the workpart can be led
automatically between the two discs and ground simultaneously on opposite sides. Advantages
of the disc grinder are good flatness and parallelism at high production rates.
The snag grinder is similar in configuration to a disc grinder. The difference is that the grinding
is done on the outside periphery of the wheel rather than on the side flat surface. The grinding
wheels are therefore different in design than those in disc grinding. Snag grinding is generally a
manual operation, used for rough grinding operations such as removing the flash from castings
and forgings, and smoothing weld joints.
Abrasive belt grinding uses abrasive particles bonded to a flexible (cloth) belt. A typical setup is
illustrated in Figure 8. Support of the belt is required when the work is pressed against it, and
this support is provided by a roll or platen located behind the belt. A flat platen is used for work
that will have a flat surface. A soft platen can be used if it is desirable for the abrasive belt to
conform to the general contour of the part during grinding. Belt speed depends on the material
being ground; a range of 750-1700 m/min (2500-5500 ft/min) is typical [3.6]. Owing to
improvements in abrasives and bonding materials, abrasive belt grinding is being used
increasingly for heavy stock removal rates, rather than light grinding, which was its traditional
application. The term belt sanding refers to the light grinding applications in which the work part
is pressed against the belt to remove burrs and high spots, and to produce an improved finish
quickly by hand.
Figure 7: Typical configuration of a disc grinder
Figure 8: Abrasive belt grinding
3.4.
Grinding Wheel Elements
3.4.1.
Abrasive Material
Different abrasive materials are appropriate for grinding different work materials. General
properties of an abrasive material used in grinding wheels include high hardness, wear
resistance, toughness, and friability. Hardness, wear resistance, and toughness are desirable
properties of any cutting-tool material. Friability refers to the capacity of the abrasive material to
fracture when the cutting edge of the grain becomes dull, thereby exposing a new sharp edge.
The development of grinding abrasives is described in our historical note. Today, the abrasive
materials of greatest commercial importance are aluminum oxide, silicon carbide, cubic boron
nitride, and diamond. They are described in Table 1, together with their relative hardness values.
3.4.2.
Grain Size
The grain size of the abrasive particle is important in determining surface finish and material
removal rate. Small grit sizes produce better finishes, while larger grain sizes permit larger
material removal rates. Thus, a choice must be made between these two objectives when
selecting abrasive grain size. 'The selection of grit size also depends to some extent on the
hardness of the work material. Harder work materials require smaller grain sizes to cut
effectively, while softer materials require larger grit sizes. Smaller grit sizes have larger numbers
and vice versa. Grain sizes used in grinding wheels typically range between 8 and 250. Grit size
8 is very coarse and size 250 is very fine. Finer grit sizes are used for lapping and super
finishing.
Table 1
Abrasive
Description
Knoop Hardness
Aluminum oxide (Al2O3)
Most common abrasive material (Section 2100
7.3.1), used to grind steel and other ferrous,
high-strength alloys.
Silicon carbide (Sic)
Warder than Al203 but not as tough 2500
(Applications include ductile metals such as
aluminum, brass, and stainless steel, as well
as brittle materials such as some cast irons
and ccitain ceramics. Cannot be used
effectively for grinding steel because of the
strong chcinical affinily between
the carbon in Sic and the iron in steel.
Boron Carbide (B4C)
Boron carbide (B4C) is expensive and is used
for lapping, cutting and grinding. Its hardness
is very close to that of diamond and heat
resistance is even better than
diamond.
Cubic Boron Nitride
Cubic boron nitride (CBN) is a synthetic
material, which also has properties very
close to that of diamond. This can cut
extremely hard materials at very high speed.
It is very expensive.
3.4.3.
Diamond
Diamonds used in cutting industries are artificial ones because of their capability to easily
fracture, during machining. Due to fracture, it presents new cutting edges rather than getting
glazed like natural diamond. Diamond is known hardest material, which cut at very high
temperature and pressure. But it is quite expensive also.
Above discussed abrasives are identified by different letters as follows. These abbreviations are
used in the specification of a grinding wheel.
(a) Aluminum oxide – A
(b) Cubic boron nitride – B
(c) Silicon carbide – C
(d) Diamond – D
3.4.4.
Grades
The grade of a grinding wheel indicates its strength which is usually represented in terms of scale
of hardness in alphabet letters. Various grades of the grinding wheels are very soft (E, F, G), soft
(H, I, J, K), medium (L, M, N, O), hard (P, Q, R, S) and very hard (T, U, W, Z). Grade of a
grinding wheel is the property attributed by the type of bonding material used in the grinding
wheel. The rate of wheel wear is less in hard wheel and more in soft wheel.
The grade of a wheel should be carefully selected according to the type of the work material. Too
hard a wheel will generate excessive heat resulting in softening of the workpiece, and too soft
wheel will be uneconomical (due to excessive wear of grinding wheel), and poor dimensional
accuracy.
Structure of a grinding wheel indicates relationship in terms of volume of abrasive grains, bond
material and voids, and their relative arrangement in a unit volume of the grinding wheel.
Grinding wheel structure is classified in three groups, and each group is sub-classified by
numbers as: dense (0, 1, 2, 3), medium dense (4, 5, 6) and open (7, 8, 9, 10, 11, 12). Proper
selection of the structure will reduce loading of the wheel by the chips, and will lead to higher
output.
3.4.5.
Bonding Materials
The bonding material holds the abrasive grains and establishes the shape and structural integrity
of the grinding wheel. Desirable properties of the bond material include strength, toughness,
hardness, and temperature resistance. The bonding material must be able to withstand the
centrifugal forces and high temperatures experienced by the grinding wheel, resist shattering in
shock loading of the wheel, and hold the abrasive grains rigidly in place to accomplish the
cutting action while allowing those grains that are worn to be dislodged so that new grains can be
exposed.
3.5.
Grinding Wheel Selection
During the selection of a grinding wheel (or deciding its specification) for particular application,
one should account for the workpiece material, workpiece requirements (in terms of tolerances,
surface finish, etc.), type of operation to be conducted, grinding conditions and type of grinder to
be used. As a rule of thumb, a soft grade wheel is recommended for the grinding of hard
materials to facilitate self sharpening action of the wheel, while relatively harder wheel is
advised for softer materials to be ground so that larger MRR can be achieved. Further, if
workpiece-wheel contact area is small, wheel wear rate will be low, and if large contact area,
wheel wear rate will be high. It is also recommended to use a close structure wheel on hard
brittle material, but a more open structure wheel for soft ductile material.
A coarse grain wheel gives rapid stock removal and rough finish, while a fine grain wheel yields
low MRR and fine finish. MRR is influenced by the method of dressing of the grinding wheel.
Bonding material should provide sufficient strength to resist various forces acting on the
grinding wheel.
3.6.
Grinding Allowance
Machine parts are processed in different machine such as lathes, shaping machines, etc. in such a
way that their final dimensions have some stock left, which is finished during the grinding
operation. The amount of this stock left is called the `grinding` allowance
3.7.
Mounting of Grinding Wheels
The proper mounting of a grinding wheel is very important. An improperly mounted wheel may
become potentially dangerous at high speeds.
The specified wheel size for the particular grinding machine to be used should not be exceeded
either in wheel diameter or in wheel width. Figure 2 illustrates a correctly mounted grinding
wheel.
Figure 9: Machine Machine Tool Grinding Machine Correctly mounted wheel
The following four items are methods and procedures for mounting grinding wheels:
•
Note that the wheel is mounted between two flanges which are relieved on their inner
surfaces so that they support the wheel only at their outer edges. This holds the wheel
more securely with less pressure and with less danger of breaking. For good support, the
range diameter should be about one-third of the wheel diameter.
•
The spindle hole in the wheel should be no more than 0.002 inch larger than the diameter
of the spindle, since a loose fit will result in difficulty in centering the wheel. If the
spindle hole is oversize, select another wheel of the proper size. If no others are available,
fit a suitable bushing over the spindle to adapt the spindle to the hole.
•
Paper blotters of the proper size usually come with The grinding wheel. If the proper
blotters are missing, cut them from heavy blotter paper (no more than 0.025-inch thick:)
and place them between the grinding wheel and each flange. The blotters must be large
enough to cover the whole area of contact between the flanges and the wheel. These
blotters serve as cushions to minimize wheel breakage.
•
When installing the grinding wheel on the wheel spindle, tighten the spindle nut firmly,
but not so. tight that undue strain will be put on the wheel.
3.8.
Dressing, Truing and Balancing
To make the glazed or loaded wheel serviceable, the wheel must be dressed and trued. Dressing
of a wheel is done to achieve one or more of the following objectives : to remove blunt abrasive
grains from the bond, to fracture the blunt grains to generate or expose sharp new cutting edges,
and to remove embedded foreign matter from the grinding wheel. However, to make the
periphery of the grinding wheel concentric to the spindle axis, it is trued.
There are various types of dressers that are used for dressing a grinding wheel, viz, Huntington
wheel dresser, dressing stick, or diamond wheel dresser. A diamond wheel dresser cuts the wheel
to shape, and is also simultaneously used for dressing and truing the wheel on a precision
grinding machine, such as surface and cylindrical grinding machines. To retain sharpness of the
diamond, it should trail the direction of rotation of the wheel at an angle between 5° to 15°, but
lead the center of rotation slightly. Traversing the diamond rapidly across the face of the wheel
will open the structure.
A grinding wheel should be accurately balanced to avoid any accident and to obtain accurate
ground parts. Out-of-balance wheel would produce vibration and a pattern on the finished
surface, and finally may lead to the damage of the spindle bearings. The balancing operation can
be carried out in two ways (static balancing and dynamic balancing). Standard procedure given
in the books should be followed for balancing of a grinding wheel.
3.9.
Summary
Abrasives on the grinding wheel are used for finishing of pre-machined surfaces. Abrasives used
for making grinding wheels are alumina, silicon carbide, cubic boron nitride and others. Abrasive
grains are mixed in various percentages with bond material to make a grinding wheel. Depending
upon the percentage of abrasive mixed in the bond material, the grinding wheel attains its
properties, for example, structure of a grinding wheel. Type of the bonding material attributes
strength to the grinding wheel. Some of the bond materials used for making a grinding wheel are
vitrified, silicate, and resinoid. Sometimes metal is also used as a bond material. As per the shape
of the workpiece to be finished, a particular type of grinding wheel is selected.
3.10.
Keywords
abrasives
grain size
grade
structure
Bond Materials
3.11.
Exercise
1. How is grinding different from other machining operations?
2. How will you specify a grinding wheel? Explain the individual elements of information
given in the specification.
3. Explain various bonding materials used in a grinding wheel. Discuss the guidelines useful
in its selection for different types of work materials.
4. What do you understand by dressing, truing and balancing of a grinding wheel?
UNIT 4
BORING, BROACHING AND SAWING MACHINE
Structure
1.1.Introduction
1.2.Objectives
1.3.Types of Boring machine
1.4.Basic Principles of Broaching
1.5.Construction and Operation of Broaching
1.6.Configuration of broaching tool
1.7.Material of broach
1.8.Different Types Of Broaches And Their Applications
1.9.Broaching Machines
1.10. Advantages and Limitations of Broaching
1.11. Summary
1.12. Keywords
1.13. Exercise
1.1.Introduction
Boring is similar to turning. It uses a single-point tool against a rotating work part. The
difference is that boring is performed on the inside diameter of an existing hole rather
than the outside diameter of an existing cylinder. In effect, boring is an internal turning
operation. Machine tools used to perform boring operations are called boring machines
(also boring mills). One might expect that boring machines would have features in
common with turning machines; indeed, as previously indicated, lathes are sometimes
used to accomplish boring.
1.2.Objectives
After studying this unit we are able to understand
− Types of Boring machine
− Basic Principles of Broaching
− Construction and Operation of Broaching
− Configuration of broaching tool
− Material of broach
− Different Types Of Broaches And Their Applications
− Broaching Machines
− Advantages and Limitations of Broaching
1.3.Types of Boring Machine
Boring mills can be horizontal or vertical. The designation refers to the orientation of the
axis of rotation of the machine spindle or work part. In a horizontal boring operation,
the setup can be arranged in either of two ways. The first setup is one in which the work
is fixtured to a rotating spindle, and the tool is attached to a cantilevered boring bar that
feeds into the work, as illustrated in Figure 1.1 (a).
FIGURE 1.1 Two forms of horizontal baring: la) boring bar is fed into a rotating
work part, and (b) work is fed past a rotating boring bar
The boring bar in this setup must be very stiff to avoid deflection and vibration during
cutting. To achieve high stiffness, boring bars are often made of cemented carbide, whose
modulus of elasticity approaches 620 x lo3 MPa (YO x 106 lb/in2). Figure 1.2 shows a
carbide boring bar.
The second possible setup is one in which the tool is mounted to a boring bar, and the
boring bar is supported and rotated between centers. The work is fastened to a feeding
mechanism that feeds it past the tool. This setup, Figure 1.1 (b), can be used to perform a
boring operation on a conventional engine lathe.
FIGURE 1 2 Boring bar made of cemented carbide
(WC-CO) that uses indexable cemented carbide inserts (Courtesy of Kennametal Inc)
A vertical boring machine (VBM) is used for large, heavy work parts with large
diameters; usually the work part diameter is greater than its length. As in Figure 1.3, the
part is clamped to a worktable that rotates relative to the machine base. Worktables up to
40 it in diameter are available. The typical boring machine can position and feed several
cutting tools simultaneously. The tools are mounted on tool heads that can be fed
horizontally and vertically relative to the worktable. One or two heads are mounted on a
horizontal cross-rail assembled to the machine tool housing above the worktable. The
cutting tools mounted above the work can be used For lacing and boring. In addition to
the tools on the cross-rail, one or two additional tool heads can be mounted on the side
columns of the housing to enable turning on the outside diameter of the work.
The tool heads used on a vertical boring machine often include turrets to accommodate
several cutting tools. This results in a loss of distinction between this machine and a
vertical turret lathe (VTL). Some machine tool builders make the distinction that the
VTL is used For work diameters up to 2.5 m (100 in), while the VAM is used for larger
diameters. Also, vertical boring mills are often applied to one-of-a-kind jobs, while
vertical turret lathes are used for batch production.
FIGURE 1.3 A vertical boring mill.
Instructional objectives
This lesson will enable the students,
(i) State and visualise the basic principle of broaching
(ii) Describe constructional features and functioning of broaching tools
(iii) Illustrate different broaching tools and their applications
(iv) Classify broaching machines w.r.t. configuration and use
(v) Identify the advantages and limitations of broaching.
1.4.Basic Principles of Broaching
Broaching is a machining process for removal of a layer of material of desired width and
depth usually in one stroke by a slender rod or bar type cutter having a series of cutting
edges with gradually increased protrusion as indicated in Fig. 1.4. In shaping, attaining
full depth requires a number of strokes to remove the material in thin layers step – by –
step by gradually infeeding the single point tool (Fig. 1.4). Whereas, broaching enables
remove the whole material in one stroke only by the gradually rising teeth of the cutter
called broach. The amount of tooth rise between the successive teeth of the broach is
equivalent to the infeed given in shaping.
Fig. 1.4 Basic principle of broaching.
Machining by broaching is preferably used for making straight through holes of various
forms and sizes of section, internal and external through straight or helical slots or
grooves, external surfaces of different shapes, teeth of external and internal splines and
small spur gears etc. Fig. 1.5 schematically shows how a through hole is enlarged and
finished by broaching.
Fig. 1.5 Schematic views of finishing hole by broaching.
1.5.Construction and Operation of Broaching
Construction of broaching tools
Construction of any cutting tool is characterized mainly by
• Configuration
• Material and
• Cutting edge geometry
1.6.Configuration of broaching tool
Both pull and push type broaches are made in the form of slender rods or bars of varying
section having along its length one or more rows of cutting teeth with increasing height
(and width occasionally). Push type broaches are subjected to compressive load and
hence are made shorter in length to avoid buckling.
The general configuration of pull type broaches, which are widely used for enlarging and
finishing preformed holes, is schematically shown in Fig. 1.6.
Fig. 1.6 Configuration of a pull type broach used for finishing holes.
The essential elements of the broach (Fig. 1.6) are :
• Pull end for engaging the broach in the machine
• Neck of shorter diameter and length, where the broach is allowed to fail, if at all, under
overloading
• Front pilot for initial locating the broach in the hole
• Roughing and finishing teeth for metal removal
• Finishing and burnishing teeth
• Rear pilot and follower rest or retriever
Broaches are designed mostly pull type to facilitate alignment and avoid buckling. The
length of the broach is governed by;
ο Type of the broach; pull or push type
ο Number of cutting edges and their pitch depending upon the work material and
maximum thickness of the material layer to be removed
ο Nature and extent of finish required.
Keeping in view that around 4 to 8 teeth remain engaged in machining at any instant, the
pitch (or gap), p, of teeth is simply decided from
where, L = length of the hole or job.
The total number of cutting teeth for a broach is estimated from,
Tn ≥ (total depth of material) / tooth rise, a1 (which is decided based on the tool – work
materials and geometry).
Broaches are generally made from solid rod or bar. Broaches of large section and
complex shape are often made by assembling replaceable separate sections or inserting
separate teeth for ease of manufacture and maintenance.
1.7.Material of broach
Being a cutting tool, broaches are also made of materials having the usual cutting tool
material properties, i.e., high strength, hardness, toughness and good heat and wear
resistance.
For ease of manufacture and resharpening the complex shape and cutting edges, broaches
are mostly made of HSS (high speed steel). To enhance cutting speed, productivity and
product quality, now-a-days cemented carbide segments (assembled) or replaceable
inserts are also used specially for stronger and harder work materials like cast irons and
steels. TiN coated carbides provide much longer tool life in broaching. Since broaching
speed (velocity) is usually quite low, ceramic tools are not used.
• Geometry of broaching teeth and their cutting edges
Fig. 1.7 shows the general configuration of the broaching teeth and their geometry. The
cutting teeth of HSS broaches are provided with positive radial or orthogonal rake (5o to
15o) and sufficient primary and secondary clearance angles (2o to 5o and 5o to 20o
respectively) as indicated in Fig. 1.7
Small in-built chip breakers are alternately provided on the roughing teeth of the broach
as can be seen in Fig. 1.7 to break up the wide curling chipsand thus preventing them
from clogging the chip spaces and increasing forces and tool wear. More ductile materials
need wider and frequent chip breakers.
Fig. 1.7 Geometry of teeth of broaching tools.
Broaching operation
Like any other machining, broaching is also accomplished through a series of following
sequential steps :
• Selection of broach and broaching machine
• Mounting and clamping the broach in the broaching machine
• Fixing workpiece in the machine
• Planning tool - work motions
• Selection of the levels of the process parameters and their setting
• Conducting machining by the broach.
• Selection of broach and broaching machine
There are various types of broaches available. The appropriate one has to be selected
based on
ο type of the job; size, shape and material
ο geometry and volume of work material to be removed from the job
ο desired length of stroke and the broach
ο type of the broaching machines available or to be used
Broaching machine has to be selected based on
ο The type, size and method of clamping of the broach to be used
ο Size, shape and material of the workpiece
ο Strength, power and rigidity required for the broaching machine to provide the desired
productivity and process capability.
• Mounting and clamping broach in the machine
The broach needs to be mounted, clamped and moved very carefully and perfectly in the
tool holding device of the broaching machine which are used for huge lot or mass
production with high accuracy and surface finish.
Pull type and push type broaches are mounted in different ways.
Fig. 1.8 typically shows a broach pull head commonly used for holding, clamping and
pulling pull type broach. Just before fitting in or removing the broach from the broach
pull head (Fig. 1.8 (a)), the sliding outer socket is pushed back against the compression
spring. After full entry of the pull end of the broach in the head the socket is brought
forward which causes locking of the broach by the radially moving strips as shown in
Fig. 1.8 (b).
Pull type broaches are also often simply and slight flexibly fitted by a suitable adapter
and pin as can be seen in Fig. 1.8
Fig. 1.8 Mounting and clamping pull type broach.
Fig. 1.9 Fitting pull type broach by an adapter and a pin.
• Mounting of workpiece or blank in broaching machine
Broaching is used for mass production and at fast rate. The blanks are repeatedly
mounted one after another in an appropriate fixture where the blanks can be easily,
quickly and accurately located, supported and clamped.
In broaching, generally the job remains fixed and the broach travels providing cutting
velocity.
Fig. 1.10 schematically shows a typical method of mounting push or pull type external
broach for through surfacing, slotting or contouring.
Fig. 1.10 Mounting external broach for surfacing and slotting.
• Tool – work motions and process variables
Any machining is associated with 2 to 5 tool – work motions as well as cutting velocity,
feed and depth of cut as process variables. But broaching operation / machine needs only
one motion which is cutting motion and is mostly imparted to the tool. In broaching feed
is provided as tooth rise. The magnitude of cutting velocity, VC is decided based on the
tool – work materials and the capability of the broaching machine. In broaching metals
and alloys, HSS broaches are used at cutting velocity of 10 to 20 m/min and carbide
broaches at 20 to 40 m/min. The value of tooth rise varies within 0.05 mm to 0.2 mm for
roughing and 0.01 to 0.04 mm for finishing teeth. Some cutting fluids are preferably used
mainly for lubrication and cooling at the chip – tool interfaces.
Fig. 1.11 typically shows mounting of blank in fixture. But occasionally the job is
travelled against the stationary broach as in continuous working type broaching machine.
Fig. 1.11 Mounting blank in broaching machine.
1.8.Different Types of Broaches and their Applications
Broaching is getting more and more widely used, wherever feasible, for high productivity
as well as product quality. Various types of broaches have been developed and are used
for wide range of applications.
Broaches can be broadly classified in several aspects such as,
• Internal broaching or External broaching
• Pull type or Push type
• Ordinary cut or Progressive type
• Solid, Sectional or Modular type
• Profile sharpened or form relieved type
Internal and external broaching (tool)
ο Internal broaching and broaches
Internal broaching tools are used to enlarge and finish various contours in through holes
preformed by casting, forging, rolling, drilling, punching etc. Internal broaching tools are
mostly pull type but may be push type also for lighter work. Pull type internal broaching
tools are generally provided with a set of roughing teeth followed by few semi-finishing
teeth and then some finishing teeth which may also include a few burnishing teeth at the
end. The wide range of internal broaching tools and their applications include;
ο through holes of different form and dimensions as indicated in fig. 1.12
ο non-circular holes and internal slots (fig. 1.12)
ο internal keyway and splines
ο teeth of straight and helical fluted internal spur gears as indicated in fig. 1.12
Fig. 1.12 Machining external gear teeth by broaching.
ο External broaching
External surface broaching competes with milling, shaping and planing and, wherever
feasible, outperforms those processes in respect of productivity and product quality.
External broaching tools may be both pull and push type.
Major applications of external broaching are :
ο un-obstructed outside surfacing; flat, peripheral and contour surfaces (fig. 1.12 (a))
ο grooves, slots, keyways etc. on through outer surfaces of objects (Fig. 1.8)
ο external splines of different forms
ο teeth of external spur gears or gear sectors as shown in Fig. 1.11 and Fig. 1.11 (b)
External broaching tools are often made in segments which are clamped in fixtures for
operation.
Fig. 1.13 Typical external broaching (a) making slot (b) teeth of gear sector
Pull type and push type broaches
During operation a pull type broach is subjected to tensile force, which helps in
maintaining alignment and prevents buckling.
Pull type broaches are generally made as a long single piece and are more widely used,
for internal broaching in particular. Push type broaches are essentially shorter in length
(to avoid buckling) and may be made in segments. Push type broaches are generally used
for external broaching, preferably, requiring light cuts and small depth of material
removal.
Ordinary – cut and Progressive type broach
Most of the broaches fall under the category of Ordinary – cut type where the teeth
increase in height or protrusion gradually from tooth to tooth along the length of the
broach. By such broaches, work material is removed in thin layers over the complete
form. Whereas, Progressive – cut type broaches have their teeth increasing in width
instead of height. Fig. 1.14 shows the working principle and configuration of such
broach.
Fig. 1.14 Progressive – cut type broaches; (a) single bar and (b) double bar type
Solid, Sectional and module type broaches
Broaches are mostly made in single pieces specially those used for pull type internal
broaching. But some broaches called sectional broaches, are made by assemblying
several sections or cutter-pieces in series for convenience in manufacturing and
resharpening and also for having little flexibility required by production in batches
having interbatch slight job variation. External broaches are often made by combining a
number of modules or segments for ease of manufacturing and handling. Fig. 1.15
typically shows solid, sectional and segmented (module) type broaches.
Fig. 1.15 (a) Solid, (b) Sectional and (c) Segmented broaches.
Profile sharpened and form relieved type broaches
Like milling cutters, broaches can also be classified as
• Profile sharpened type broaches;
Such cutters have teeth of simple geometry with same rake and clearance angles all over
the cutting edge. These broaches are generally designed and used for machining flat
surface(s) or circular holes.
• Form relieved type broaches
These broaches, being used for non-uniform profiles like gear teeth etc., have teeth where
the cutting edge geometry is more complex and varies point – to – point along the cutting
edges. Here the job profile becomes the replica of the tool form. Such broaches are
sharpened and resharpened by grinding at their rake faces unlike the profile sharpened
broaches which are ground at the flank surfaces.
1.9.Broaching Machines
The unique characteristics of broaching operation are
• For producing any surface, the form of the tool (broach) always provides the Generatrix
and the cutting motion (of the broach relative to the job surface) provides the Directrix.
• So far as tool – work motions, broaching needs only one motion and that is the cutting
motion (velocity) preferably being imparted to the broach.
Hence design, construction and operation of broaching machines, requiring only one such
linear motion, are very simple. Only alignments, rigidity and reduction of friction and
wear of slides and guides are to be additionally considered for higher productivity,
accuracy and surface finish.
Broaching machines are generally specified by
ο Type; horizontal, vertical etc.
ο Maximum stroke length
ο Maximum working force (pull or push)
ο Maximum cutting velocity possible
ο Power
ο Foot print
Most of the broaching machines have hydraulic drive for the cutting motion. Electromechanical drives are also used preferably for high speed of work but light cuts.
There are different types of broaching machines which are broadly classified
• According to purpose of use
∆ general purpose
∆ single purpose
∆ special purpose
• According to nature of work
∆ internal broaching
∆ external (surface) broaching
• According to configuration
∆ horizontal
∆ vertical
• According to number of slides or stations
∆ single station type
∆ multiple station type
∆ indexing type
• According to tool / work motion
∆ intermittent (one job at a time) type
∆ continuous type
Some of the broaching machines of common use have been discussed here.
ο Horizontal broaching machine
Horizontal broaching machines, typically shown in Fig. 1.16, are the most versatile in
application and performance and hence are most widely employed for various types of
production. These are used for internal broaching but external broaching work are also
possible. The horizontal broaching machines are usually hydraulically driven and
occupies large floor space.
Fig. 1.16 Horizontal broaching machine.
ο Vertical broaching machine
Vertical broaching machines, typically shown in Fig. 1.17,
∆ occupies less floor space
∆ are more rigid as the ram is supported by base
∆ mostly used for external or surface broaching though internal broaching is also possible
and occasionally done.
Fig. 1.17 Vertical broaching machine.
ο High production broaching machines
Broaching operation and broaching machines are as such high productive but its speed of
production is further enhanced by;
∆ incorporating automation in tool – job mounting and releasing
∆ increasing number of workstations or slides for simultaneous multiple production
∆ quick changing the broach by turret indexing
∆ continuity of working
Fig. 1.18 schematically shows the principle and methods of continuous broaching, which
is used for fast production of large number of pieces by surface broaching.
Fig. 1.18 Continuous broaching
1.10.
Advantages and Limitations of Broaching
Major advantages
• Very high production rate (much higher than milling, planing, boring etc.)
• High dimensional and form accuracy and surface finish of the product
• Roughing and finishing in single stroke of the same cutter
• Needs only one motion (cutting), so design, construction, operation and control are
simpler
• Extremely suitable and economic for mass production
Limitations
• Only through holes and surfaces can be machined
• Usable only for light cuts, i.e. low chip load and unhard materials
• Cutting speed cannot be high
• Defects or damages in the broach (cutting edges) severely affect product quality
• Design, manufacture and restoration of the broaches are difficult and expensive
• Separate broach has to be procured and used whenever size, shape and geometry of the
job changes
• Economic only when the production volume is large.
1.11.
Summary
In this unit we have studied Boring machines ; Types of Boring machine ; Boring haps
and heads; Various operations using boring heads; Boring operations using end supports;
Introduction to Broaching machine ; Types of Broaching machine; Broaching tool
nomenclature; Types of Broaches; Broaching options compared with other process
(advantages & limitations.); External; Lubrication and cooling; Application of Broaching
1.12.
Keywords
− Vertical Boring Machine
− Vertical Turret Lathe
− Internal Broaching And Broaches
− External Broaching
− Horizontal Broaching Machine
− Broaching Machines
1.13.
Exercise
1. What are the different types of boring machines?
2. Explain the basic principles of broaching.
3. What are the advantages and limitations of broaching?
UNIT 1
GEAR MANUFACTURING
Structure
1.1.Introduction
1.2.Objectives
1.3.Materials for Gears
1.4.Different methods of Gear manufacturing
1.5.Forming Gear Teeth
1.6.Machining
1.7.Gear shaping
1.8.Gear Hibbing
1.9.Gear finishing process
1.10.
Grinding
1.11.
Summary
1.12.
Keywords
1.13.
Exercise
1.1.Introduction
Gear manufacturing can be divided into two categories namely forming and machining as
shown in flow chart in Fig 1.1. Forming consists of direct casting, molding, drawing, or
extrusion of tooth forms in molten, powdered, or heat softened materials and machining
involves roughing and finishing operations. They are discussed in the different sections
of this chapter.
1.2.Objectives
After studying this unit we are able to understand
− Gear tooth element
− Materials for Gears
− Different methods of Gear manufacturing
− Gear generating methods
− Gear milling
− Gear shaping
− Gear Hibbing
− Gear finishing process
1.3.Materials for Gears
The various materials used for gears include a wide variety of cast irons, non ferrous
material &non - material materials the selection of the gear material depends upon:
•
Type of service
•
Peripheral speed
•
Degree of accuracy required
•
Method of manufacture
•
Required dimensions & weight of the drive
•
Allowable stress
•
Shock resistance
•
Wear resistance.
1) Cast iron is popular due to its good wearing properties, excellent machinability & Ease
of producing complicated shapes by the casting method. It is suitable where large gears
of complicated shapes are needed.
2) Steel is sufficiently strong & highly resistant to wear by abrasion.
3) Cast steel is used where stress on gear is nigh & ills difficult to fabricate the gears.
4) Plain carbon steels find application for industrial gears where high toughness
combined with high strength.
5) Alloy steels are used where high tooth strength & low tooth wear are required.
6) Aluminum is used where low inertia of rotating mass is desired.
7) Gears made of non — Metallic materials give noiseless operation at high peripheral
speeds.
1.4.Different Methods of Gear Manufacturing
Fig. : 1.1 Categories of gear manufacturing process
1.5.Forming Gear Teeth
Characteristics: In all tooth-forming operations, the teeth on the gear are formed all at
once from a mold or die into which the tooth shapes have been machined. The accuracy
of the teeth is entirely dependent on the quality of the die or mold and in general is much
less than that can be obtained from roughing or finishing methods. Most of these methods
have high tooling costs making them suitable only for high production quantities. The
various forming techniques are discussed below in detail:
Casting
Sand casting, die casting and investment casting are the casting processes that are best
suited for gears and are shown in fig 1.2. They are explained in the following sections:
Fig.1.2 Casting processes
a. Sand Casting
Characteristics:
− The characteristics of sand cast gears are,
− Cheaper low quality gear in small numbers
− The tooling costs are reasonable
− Poor Surface finish and dimensional accuracy
− Due to low precision and high backlash, they are noisy.
− They are suited for non- critical applications
Applications: (without finishing operation)
Sand casting is used for gear manufacture which are used in variety of applications such
as for toys, small appliances, cement-mixer barrels, hoist gearbox of dam gate lifting
mechanism, hand operated crane etc.,
Materials:
The materials that can be sand cast are C I, cast steel, bronzes, brass and ceramics. The
process is confined to large gears that are machined later to required accuracy.
(a)
(b)
Fig 1.3(a) SAE 4640 cast steel helical gear, (b) Silicon bronze heavy duty drive gears
from 200mm to 1600mm diameter
b. Die casting
Characteristics:
The characteristics of die cast gears are,
1. Better surface finish and accuracy (tooth spacing and concentricity)
2. High tooling costs
3. Suited for large scale production Applications:
Applications:
Gears that are die cast are used in instruments, cameras, business machines, washing
machines, gear pumps, small speed reducers, and lawn movers. Fig. 5.3 shows gears that
are manufactured by die casting.
Materials:
Materials used to manufacture these gears are zinc, aluminium and brass. The gears made
from this process are not used for high speeds and heavy tooth loading. They are
normally applied for small size gears.
c. Investment casting or lost wax process
Characteristics:
The characteristics of gears that are manufactured by investment casting are,
1.
Reasonably accurate gears
2.
Applicable for a variety of materials
3.
Refractory mould material
4.
Allows high melt-temperature materials
5.
Accuracy depends on the original master pattern used for the mold.
Materials:
Tool steel, nitriding steel, monel, beryllium copper are the materials that can be
investment casted for the manufacture of gears. The process is used only if no other
process is suitable since production cost is high. Fig 1.4 shows a wire twister stellite gear
which mates with a rack made by IC. Complicated shape makes it economical to produce
by investment casting process.
Fig. 1.4 Complicated shape of gear manufactured by Investment casting
d. Sintering or P/M process:
The powder metallurgy technique used for gear manufacture is shown in fig 1.5.
Characteristics:
1.
Accuracy similar to die-cast gears
2.
Material properties can be Tailor made
3.
Typically suited for small sized gears
4.
Economical for large lot size only
Fig 1.5 Process chart for P/M gear manufacture
As shown in Fig 1.6, for the components manufactured by P/M technique, secondary
machining is not required. Fig 1.7 shows cluster gears, different types of gears that can be
combined and keyways can be built-in.
Fig. 1.6. Components manufactured by sintering
Fig1.7 Cluster gears, combination of gears and gears with key ways
Fig 1.8 shows helical gears and combination of gears made by P/M or sintering process.
Material utilization is more than 95% in this manufacturing process. The material
utilizations of forged and sintered processes are shown in Fig 1.9.
Fig 1.8 Helical gears and combination of gears
Fig. 1.9 Material utilization of forged and sintered processes
Fig 1.10 shows the P/M gear production by hot forging process and the manufactured
components are shown in fig 1.11.
Fig 1.10 P/M gear production by hot forging process
(a)
(b)
Fig 1.11 P/M gears by hot forging process
Injection Molding
Injection molding is used to make nonmetallic gears in various thermoplastics such as
nylon and acetal. These are low precision gears in small sizes but have the advantages of
low cost and the ability to be run without lubricant at light loads.
Applications:
Injection molded gears are used in cameras, projectors, wind shield wipers, speedometer,
lawn sprinklers, washing machine. They are shown in fig.1.12 and 1.13.
Materials:
The materials for injection molding components are Nylon, cellulose acetate,
polystyrene, polyimide, phenolics.
Fig. 1.12 IM camera gears
Fig 1.13 Food mixer
Fig 1.14 Compression molded gear
Extruding
Extruding is used to form teeth on long rods, which are then cut into usable lengths and
machined for bores and keyways etc. Nonferrous materials such as aluminum and copper
alloys are commonly extruded rather than steels. This result in good surface finishes with
clean edges and pore free dense structure with higher strength. Table 5.1 shows various
extruded sections along with their number of teeth, outside diameter, pitch diameter, root
diameter, pitch and tooth thickness.
Fig. 1.15 Extruded gears
Materials:
Aluminum, copper, naval brass, architectural bronze and phosphor bronze are the
materials that are commonly extruded.
Applications:
Splined hollow & solid shafts, sector gears are extruded and various gears are shown in
fig 1.15.
Table 1.1 Specifications of various extruded sections
The progression in the formation of a gear blank by cold forming is shown in fig 1.16 and
the stages in the extrusion of a gear is shown in fig 1. 17.
Fig 1.16 Progression of a cold formed gear blank
Fig 1.17 Stages in extrusion of a gear
Helical gears manufactured by extrusion are shown below in fig 1.18.
Fig 1.18 Helical gear made by extrusion
Cold Drawing:
Cold drawing forms teeth on steel rods by drawing them through hardened dies. The cold
working increases strength and reduces ductility. The rods are then cut into usable lengths
and machined for bores and keyways, etc.
Fig 1.19 For cold drawing, the 11-tooth pinion below is enlarged by AGMA-ASA
standard to the form above, avoiding undercut and giving radius rather than sharp corners
Stamping:
Sheet metal can be stamped with tooth shapes to form low precision gears at low cost in
high quantities. The surface finish and accuracy of these gears are poor.
Applications:
Stamped gears are used as toy gears, hand operated machine gears for slow speed
mechanism.
Precision stamping:
In precision stamping, the dies are made of higher precision with close tolerances
wherein the stamped gears will not have burrs.
Applications:
Clock gears, watch gears etc.
Preforming
For close die forging the feed stock has to be very near to the net shape and this is
obtained by performing. This is explained by flow diagrams both in sinter forging and
precision hot forging.
Forging:
The steps in forging process are represented in fig 1.20 and the forged gears are shown in
fig 1.21.
Fig 1.20 Procedure for forging of gears
Fig 1.21 Various forged gears
1.6.Machining
The bulk of power transmitting metal gears of machinery are produced by machining
process from cast, forged, or hot rolled blanks. Refer fig 1.1 for classification of
machining processes. Roughing processes include milling the tooth shape with formed
cutters or generating the shape with a rack cutter, a shaping cutter or a hob cutter which
are shown in fig 1.22.
Fig 1.22 Various gear cutters
Despite its name, the roughing processes actually produce a smooth and accurate gear
tooth. Only for high precision and quiet running, the secondary finishing operation is
justified at added cost.
Roughing processes
Roughing process consists of forming, generation, shaping and hobbing processes. By
this method gears are made to an accuracy which is more than adequate for the slow
speed operations. These processes are dealt here.
Form milling
Forming is sub-divided into milling by disc cutters and milling by end mill cutter which
are having the shape of tooth space.
Form milling by disc cutter:
The disc cutter shape conforms to the gear tooth space. Each gear needs a separate cutter.
However, with 8 to 10 standard cutters, gears from 12 to 120 teeth can be cut with fair
accuracy. Tooth is cut one by one by plunging the rotating cutter into the blank as shown
in fig 1.23.
Fig 1.23 Form milling by disc cutter
Form milling by end mill cutter:
The end mill cutter shape conforms to tooth spacing. Each tooth is cut at a time and then
indexed for next tooth space for cutting. A set of 10 cutters will do for 12 to 120 teeth
gears. It is suited for a small volume production of low precision gears. The form milling
by end mill cutter is shown in fig 1.24.
Fig 1.24 Form milling by end mill cutter
To reduce costs, the same cutter is often used for the multiple-sized gears resulting in
profile errors for all but one number of teeth. Form milling method is the least accurate of
all the roughing methods.
Rack generation:
In rack cutter the tooth shape is trapezoid and can be made easily. The hardened and
sharpened rack is reciprocated along the axis of the gear blank and fed into it while gear
blank is being rotated so as to generate the involute tooth on the gear blank as shown in
fig 1.25.
Fig 1.25 Generation of involute tooth on gear blank
The rack and gear blank must be periodically repositioned to complete the circumference.
This introduces errors in the tooth geometry making this method less accurate than
shaping and hobbing.
(a)
(b)
Fig 1.26 (a) (b) Rack generations
The process is limited to small gears since the length of the rack has to be equal to
circumference of the gear at pitch diameter. The generation of spur gear by planning is
shown in fig 1.27.
Fig 1.27 generation of spur gear by planning
1.7.Gear Shaping
Gear shaping used a cutting tool in the shape of a gear which is reciprocated axially
across the gear blank to cut the teeth while the blank rotates around the shaper tool. It is a
true shape-generation process in which the gear-shaped tool cuts itself into mesh with the
gear blank as shown in fig 5.28. The accuracy is good, but any errors in one tooth of the
shaper cutter will be directly transferred to the gear. Internal gears can be cut with this
method as well.
Fig 1.28 Gear shaping
1.8.Hobbing
Hob teeth are shaped to match the tooth space and are interrupted with grooves to provide
cutting surfaces. It rotates about an axis normal to that of the gear blank, cutting into the
rotating blank to generate the teeth as shown in fig 1.29.
It is the most accurate of the roughing processes since no repositioning of tool or blank is
required and each tooth is cut by multiple hob-teeth, averaging out any tool errors.
Excellent surface finish is achieved by this method and it is widely used for production of
gears.
Fig 1.29 Hobbing
1.9.Finishing Processes
When high precision is required secondary operation can be performed to gears made by
any of the above roughing methods. Finishing operations typically removes little or no
material but improves dimensional accuracy, surface finish, and or hardness. The various
finishing processes are shown in fig 1.1.
Shaving:
Shaving is similar to gear shaping, but uses accurate shaving tools to remove small
amounts of material from a roughed gear to correct profile errors and improve surface
finish. Shaving operation is shown in fig 1.30.
Fig 1.30 External gear being shaved
1.10.
Grinding
In grinding, a contoured grinding wheel is run over machined surface of the gear teeth
using computer control. With a small amount of metal removal high surface finish is
obtained. Fig 1.31 shows grinding operations and dressing of the wheel.
Fig 1.31 (a) Grinding the flanks only, (b) Grinding root and flanks, (c) Grinding each
flank separately with twin grinding wheels and (d) Pantograph dressing of the wheel
Grinding is used to correct the heat-treatment distortion in gears hardened after roughing.
Improvement in surface finish and error correction of earlier machining are added
advantages. Grinding operation for gears can be done by profile grinding or form
grinding as shown in fig 1.32 and 1.33.
(a)
(b)
Fig 1.32 (a) Maag zero pressure angle profile grinding and (b) Maag profile grinding
Fig 1.33 David Brown form grinding of worm threads
Burnishing:
In burnishing, a specially hardened gear is run over rough machined gear. The high forces
at the tooth interface cause plastic yielding of the gear tooth surface which improves
finish and work hardens the surface creating beneficial compressive residual stresses.
Lapping and Honing:
Lapping and honing both employ an abrasive-impregnated gear or gear-shaped tool that
is run against the gear to abrade the surface. In both cases, the abrasive tool drives the
gear in what amounts to an accelerated and controlled run-in to improve surface finish
and the accuracy. Fig 1.34 shows lapping operation for bevel gears.
Fig 1.34 Special bevel gears being lapped
Quality of the Gear:
The quality of gear gives its accuracy, dimensional and profile which dictates the
suitability of gears for different operations.
Various standards for assuring the quality of gears are,
•
The AGMA standard 2000-A88 defines dimension tolerance for gear teeth and a
quality index Qv that ranges from the lowest quality 3 to the highest precision 16.
•
DIN 3962 defines quality index in another way. Highest quality is assigned number 1
and the lowest quality is assigned number 12.
Based on the machining/production techniques the accuracy of gears varies viz., with the
pitch error, profile errors and surface finish, the Qv varies. These errors give rise to
vibration in the gears and affect their smooth running. Consequently the gear quality
limits their speed of operation. The various gear manufacturing processes and the
corresponding dynamic load factors at various speeds are depicted in Fig. 1.35. The
limiting speeds and dynamic load factors for various quality of gears is shown in Fig.
1.36
Fig. 1.35 Various gear manufacturing processes, their operating speed limits and dynamic
load factors
Fig. 1.36 Gear quality, their limiting speeds and dynamic load factors
Table 1.2 Allowable velocities and applications of gears of various accuracy grades
Summary
1.11.
In this unit we have studied, Different methods of Gear manufacturing, Forming Gear
Teeth, Machining, Gear shaping, Gear Hibbing, Gear finishing process, Grinding
Keywords
1.12.
Machining
Gear shaping
Gear Hibbing
1.13.
Exercise
1. What are different methods of gear manufacturing.
2. How to form a gear teeth
3. Write short note on:
a. Machining
b. Gear shaping
c. Gear Hibbing
d. Gear finishing process
e. Grinding
UNIT 2
METAL FINISHING PROCESS
Structure
2.1.
Introduction
2.2.
Objectives
2.3.
Lapping
2.4.
Honing
2.5.
Superfinishing
2.6.
Super finishing process Burnishing - Polishing - Buffing
2.7.
Application of super finishing operations
2.8.
Summary
2.9.
Keywords
2.10.
Exercise
2.1.
Introduction
In casting process the molten metal is poured into a mould cavity. Therefore suitability of
a casting operation depends on the selection of an appropriate moulding process and
mould material. Suitability of a moulding material depends upon the type of material
being poured, number of castings being made, the type of casting, quality requirement by
the customer and finally on the mould and core making equipment owned by the foundry
2.2.
Objectives
At the end of this lesson the students would be able to
(i) understand the significance of superfinishing process
(ii) state various applications of the superfinishing process
(iii) illustrate various techniques of superfinishing process
To ensure reliable performance and prolonged service life of modern machinery, its
components require to be manufactured not only with high dimensional and geometrical
accuracy but also with high surface finish. The surface finish has a vital role in
influencing functional characteristics like wear resistance, fatigue strength, corrosion
resistance and power loss due to friction. Unfortunately, normal machining methods like
turning, milling or even classical grinding can not meet this stringent requirement.
Table 2.1 illustrates gradual improvement of surface roughness produced by various
processes ranging from precision turning to superfinishing including lapping and honing.
Table 2.1
Therefore, superfinishing processes like lapping, honing, polishing, burnishing are being
employed to achieve and improve the above-mentioned functional properties in the
machine component.
2.3.
Lapping
Lapping is regarded as the oldest method of obtaining a fine finish. Lapping is basically
an abrasive process in which loose abrasives function as cutting points finding
momentary support from the laps. Figure 3.1 schematically represents the lapping
process. Material removal in lapping usually ranges from .003 to .03 mm but many reach
0.08 to 0.1mm in certain cases.
Characteristics of lapping process:
1. Use of loose abrasive between lap and the workpiece
2. Usually lap and workpiece are not positively driven but are guided in contact with
each other
3. Relative motion between the lap and the work should change continuously so that
path of the abrasive grains of the lap is not repeated on the workpiece. Fig. 3.1 Scheme of
lapping process
Fig. 3.1 Scheme of lapping process
Cast iron is the mostly used lap material. However, soft steel, copper, brass, hardwood as
well as hardened steel and glass are also used.
Abrasives of lapping:
• Al2O3 and SiC, grain size 5~100µm
• Cr2O3, grain size 1~2 µm
• B4C3, grain size 5-60 µm
• Diamond, grain size 0.5~5 V
Vehicle materials for lapping
• Machine oil
• Rape oil
• grease
Technical parameters affecting lapping processes are:
• unit pressure
• the grain size of abrasive
• concentration of abrasive in the vehicle
• lapping speed
Lapping is performed either manually or by machine. Hand lapping is done with abrasive
powder as lapping medium, whereas machine lapping is done either with abrasive
powder or with bonded abrasive
2.3.1 Hand lapping
Hand lapping of flat surface is carried out by rubbing the component over accurately
finished flat surface of master lap usually made of a thick soft close-grained cast iron
block. Abrading action is accomplished by very fine abrasive powder held in a vehicle.
Manual lapping requires high personal skill because the lapping pressure and speed have
to be controlled manually.
Laps in the form of ring made of closed grain cast iron are used for manual lapping of
external cylindrical surface. The bore of the ring is very close to size of the workpiece
however, precision adjustment in size is possible with the use of a set screw as illustrated
in Fig.30.2(a). To increase range of working, a single holder with interchangeable ring
laps can also be used. Ring lapping is recommended for finishing plug gauges and
machine spindles requiring high precision. External threads can be also lapped following
this technique. In this case the lap is in the form of a bush having internal thread.
Fig. 2.2 Manual Ring lapping of external cylindrical surface Fig. 2.2 (b) Manual
Lapping of internal cylindrical surfaces
Solid or adjustable laps, which are ground straight and round, are used for lapping holes.
For manual lapping, the lap is made to rotate either in a lathe or honing machine, while
the workpiece is reciprocated over it by hand. Large size laps are made of cast iron, while
those of small size are made of steel or brass. This process finds extensive use in
finishing ring gauges.
2.3.2 Lapping Machine
Machine lapping is meant for economic lapping of batch qualities. In machine lapping,
where high accuracy is demanded, metal laps and abrasive powder held in suitable
vehicles are used. Bonded abrasives in the form wheel are chosen for commercial
lapping. Machine lapping can also employ abrasive paper or abrasive cloth as the lapping
medium. Production lapping of both, flat and cylindrical surfaces are illustrated in Fig.
30.3 (a) and (b). In this case cast iron plate with loose abrasive carried in a vehicle can be
used. Alternatively, bonded abrasive plates may also be used. Centreless, roll lapping
uses two cast iron rolls, one of which serves as the lapping roller twice in diameter than
the other one known as the regulating roller. During lapping the abrasive compound is
applied to the rolls rotating in the same direction while the workpiece is fed across the
rolls. This process is suitable for lapping a single piece at a time and mostly used for
lapping plug gauges, measuring wires and similar straight or tapered cylindrical parts.
Fig.2.3 Production lapping on (a) flat surface (b) cylindrical surface
Centreless lapping is carried out in the same principle as that of centreless grinding. The
bonded abrasive lapping wheel as well as the regulating wheel are much wider than those
used in centreless grinding. This technique is used to produce high roundness accuracy
and fine finish, the workpiece requires multi-pass lapping each with progressively finer
lapping wheel. This is a high production operation and suitable for small amount of
rectification on shape of workpiece. Therefore, parts are to be pre-ground to obtain
substantial straightness and roundness. The process finds use in lapping piston rings,
shafts and bearing races.
Machines used for lapping internal cylindrical surfaces resembles honing machines used
with power stroke. These machines in addition to the rotation of the lap also provide
reciprocation to the workpiece or to the lap. The lap made usually of cast iron either solid
or adjustable type can be conveniently used.
Figure 2.4 shows that to maximize the MRR (material removal rate) an optimum lapping
pressure and abrasive concentration in the vehicle have to be chosen.
Fig. 2.4 Effect of abrasive content on MRR Fig. 30.5 Effect of lapping pressure on
surface roughness and MRR
The effect of unit pressure on MRR and surface roughness is shown in Fig. 2.5. It is
shown in the same figure that unit pressure in the range of p1-p2 gives the best values for
MRR and roughness of the lapped surface.
The variation in MRR and surface roughness with grain size of abrasive are shown in
Fig.2.6. It appears that grain size corresponding to permissible surface roughness and
maximum MRR may be different. Primary consideration is made on the permissible
surface roughness in selecting abrasive grain size.
Fig. 2.6 Effect of abrasive grain size on surface roughness and MRR Fig. 2.7 Effect of
lapping time on surface roughness and MRR
The dependence of MRR, surface roughness and linear loss (L) of workpiece dimension
is shown in fig. 2.7. Lapping conditions are so chosen that designed surface finish is
obtained with the permissible limit of linear loss of workpiece dimension as shown in
Fig. 2.8.
Fig. 2.8 Criteria for choosing lapping time
2.4.
Honing
Honing is a finishing process, in which a tool called hone carries out a combined rotary
and reciprocating motion while the workpiece does not perform any working motion.
Most honing is done on internal cylindrical surface, such as automobile cylindrical walls.
The honing stones are held against the workpiece with controlled light pressure. The
honing head is not guided externally but, instead, floats in the hole, being guided by the
work surface (Fig. 2.9). It is desired that
1. honing stones should not leave the work surface
2. stroke length must cover the entire work length.
In honing rotary and oscillatory motions are combined to produce a cross hatched lay
pattern as illustrated in Fig. 2.10
Fig. 2.9 Honing tool Fig. 2.10 Lay pattern produced by combination of rotary and
oscillatory motion
The honing stones are given a complex motion so as to prevent every single grit from
repeating its path over the work surface. The critical process parameters are:
1. rotation speed
2. oscillation speed
3. length and position of the stroke
4. honing stick pressure
With conventional abrasive honing stick, several strokes are necessary to obtain the
desired finish on the work piece. However, with introduction of high performance
diamond and cBN grits it is now possible to perform the honing operation in just one
complete stroke. Advent of precisely engineered microcrystalline cBN grit has enhanced
the capability further. Honing stick with microcrystalline cBN grit can maintain sharp
cutting condition with consistent results over long duration.
Superabrasive honing stick with monolayer configuration (Fig. 2.11), where a layer of
cBN grits are attached to stick by a galvanically deposited metal layer, is typically found
in single stroke honing application.
Fig.2.11 Superabrasive honing stick with single layer configuration
With the advent of precision brazing technique, efforts can be made to manufacture
honing stick with single layer configuration with a brazed metal bond. Like brazed
grinding wheel such single layer brazed honing stick are expected to provide controlled
grit density, larger grit protrusion leading to higher material removal rate and longer life
compared to what can be obtained with a galvanically bonded counterpart.
The important parameters that affect material removal rate (MRR) and surface roughness
(R) are:
(i) unit pressure, p
(ii) peripheral honing speed, Vc
(iii) honing time, T
The variation of MRR (Q) and R with unit pressure is shown in Fig. 2.12. It is evident
from the graph that the unit pressure should be selected so as to get minimum surface
roughness with highest possible MRR.
Fig. 2.12: Effect of honing pressure on MRR and surface finish
Figure 2.13 shows that an increase of peripheral honing speed leads to enhancement of
material removal rate and decrease in surface roughness.
Figure 2.14 shows that with honing time T, MRR decreases. On the other hand, surface
roughness decreases and after attaining a minimum value again rises. The selection of
honing time depends very much on the permissible surface roughness.
Fig. 2.13 Effect of peripheral honing speed Fig. 2.14 Effect of honing time on material
removal rate and surface roughness
2.5.
Superfinishing
Figure 2.15 illustrates superfinishing end-face of a cylindrical workpiece. In this both
feeding and oscillation of the superfinishing stone is given in the radial direction.
Figure 2.16 shows the superfinishing operation in plunge mode. In this case the abrasive
stone covers the section of the workpiece requiring superfinish. The abrasive stone is
slowly fed in radial direction while its oscillation is imparted in the axial direction.
Fig. 2.15 superfinishing of end face of a cylindrical work piece in radial mode Fig. 2.16
superfinishing operation in plunge mode
Superfinishing can be effectively done on a stationary workpiece as shown in Fig. 2.17.
In this the abrasive stones are held in a disc which oscillates and rotates about the axis of
the workpiece.
Fig. 2.18 shows that internal cylindrical surfaces can also be superfinished by axially
oscillating and reciprocating the stones on a rotating workpiece.
Fig. 2.17 Abrasive tool rotating and oscillating about a stationary workpiece
Fig. 2.17 Abrasive tool rotating and oscillating about a stationary workpiece Fig. 2.18
Superfinishing of internal surface
2.5.1 Burnishing
The burnishing process consists of pressing hardened steel rolls or balls into the surface
of the workpiece and imparting a feed motion to the same. Ball burnishing of a
cylindrical surface is illustrated in Fig. 2.19.
Fig. 2.19 Scheme of ball burnishing
During burnishing considerable residual compressive stress is induced in the surface of
the work piece and thereby fatigue strength and wear resistance of the surface layer
increase.
Magnetic float polishing
Magnetic float polishing (Fig.2.20) finds use in precision polishing of ceramic balls. A
magnetic fluid is used for this purpose. The fluid is composed of water or kerosene
carrying fine ferro-magnetic particles along with the abrasive grains. Ceramic balls are
confined between a rotating shaft and a floating platform. Abrasive grains ceramic ball
and the floating platform can remain in suspension under the action of magnetic force.
The balls are pressed against the rotating shaft by the float and are polished by their
abrasive action. Fine polishing action can be made possible through precise control of the
force exerted by the abrasive particles on the ceramic ball.
Fig. 2.20 Scheme of magnetic float polishing
Magnetic field assisted polishing
Magnetic field assisted polishing is particularly suitable for polishing of steel or ceramic
roller. The process is illustrated schematically in Fig. 30.21. A ceramic or a steel roller is
mounted on a rotating spindle. Magnetic poles are subjected to oscillation, thereby,
introducing a vibratory motion to the magnetic fluid containing this magnetic and
abrasive particles. This action causes polishing of the cylindrical roller surface. In this
technique, the material removal rate increases with the field strength, rotational speed of
the shaft and mesh number of the abrasive. But the surface finish decreases with the
increase of material removal rate.
Fig. 2.21 scheme of magnetic field assisted polishing
Electropolishing
Electropolishing is the reverse of electroplating. Here, the workpiece acts as anode and
the material is removed from the workpiece by electrochemical dissolution. The process
is particularly suitable for polishing irregular surface since there is no mechanical contact
between workpiece and polishing medium. The electrolyte electrochemically etches
projections on the workpiece surface at a faster rate than the rest, thus producing a
smooth surface. This process is also suitable for deburring operation.
2.6.
Summary
In this unit we have studied Honing; Description and construction of honing tool.;
Application of honing process; Lopping; Description of Lapping compound and tool;
Application of Lapping ; Super finishing process Burnishing - Polishing - Buffing ;
Application of super finishing operations.
2.7.
Keywords
Lapping
Honing
Superfinishing
Buffing
Burnishing
2.8.
Exercise
1. How is the size of the abrasive grain chosen?
2. Can cBN be used in honing stick in single layer configuration?
3. How does superfinishing differ from honing?
4. State the advantage of electro polishing over mechanical polishing.
5. How is the surface quality improved in ball burnishing?
Unit 3
Pattern Making
Structure
3.1.Introduction
3.2.Objectives
3.3.Functions of the Pattern
3.4.Pattern Materials
3.5.Pattern Allowances
3.6.Types of Patterns
3.6.1. Solid or Single Piece Pattern
3.6.2. Split Pattern
3.6.3. Match Plate Pattern
3.6.4. Cope and Drag Pattern
3.6.5. Loose Piece Pattern
3.6.6. Gated Pattern
3.6.7. Sweep Pattern
3.6.8. Skeleton Pattern
3.6.9. Segmental Pattern
3.6.10. Follow Board Pattern
3.6.11. Lagged-up Pattern
3.6.12. Shell Pattern
3.6.13. Left and Right hand Pattern
3.7.Core Boxes
3.8.Summary
3.9.Keywords
3.10.
Exercise
3.1.Introduction
The pattern is a physical model of the casting used to make the mold. The mold is made
by packing some readily formed aggregate material, such as molding sand, around the
pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is
ultimately filled with metal to become the casting. If the casting is to be hollow, as in the
case of pipe fittings, additional patterns, referred to as cores, are used to form these
cavities.
Pattern
The pattern is the principal tool during the casting process. It is the replica of the object to
be made by the casting process, with some modifications. The main modifications are the
addition of pattern allowances, and the provision of core prints. If the casting is to be
hollow, additional patterns called cores are used to create these cavities in the finished
product. The quality of the casting produced depends upon the material of the pattern, its
design, and construction. The costs of the pattern and the related equipment are reflected
in the cost of the casting. The use of an expensive pattern is justified when the quantity of
castings required is substantial.
3.2.Objectives
After studying this unit we are able to understand
− Functions of the Pattern
− Pattern Materials
− Pattern Making Tools
− Pattern Allowances
− Types of Patterns
3.3.Functions of the Pattern
1. A pattern prepares a mold cavity for the purpose of making a casting.
2. A pattern may contain projections known as core prints if the casting requires a core
and need to be made hollow.
3. Runner, gates, and risers used for feeding molten metal in the mold cavity may form a
part of the pattern.
4. Patterns properly made and having finished and smooth surfaces reduce casting
defects.
5. A properly constructed pattern minimizes the overall cost of the castings.
3.4.Pattern Material
Patterns may be constructed from the following materials. Each material has its own
advantages, limitations, and field of application. Some materials used for making patterns
are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and
resins. To be suitable for use, the pattern material should be:
1.
Easily worked, shaped and joined
2.
Light in weight
3.
Strong, hard and durable
4.
Resistant to wear and abrasion
5.
Resistant to corrosion, and to chemical reactions
6.
Dimensionally stable and unaffected by variations in temperature and humidity
7.
Available at low cost
The usual pattern materials are wood, metal, and plastics. The most commonly used
pattern material is wood, since it is readily available and of low weight. Also, it can be
easily shaped and is relatively cheap. The main disadvantage of wood is its absorption of
moisture, which can cause distortion and dimensional changes. Hence, proper seasoning
and upkeep of wood is almost a pre-requisite for large-scale use of wood as a pattern
material.
Figure 3.1: A typical pattern attached with gating and risering system
3.5.Pattern Allowances
Pattern allowance is a vital feature as it affects the dimensional characteristics of the
casting. Thus, when the pattern is produced, certain allowances must be given on the
sizes specified in the finished component drawing so that a casting with the particular
specification can be made. The selection of correct allowances greatly helps to reduce
machining costs and avoid rejections. The allowances usually considered on patterns and
core boxes are as follows:
1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance
4. Distortion or camber allowance
5. Rapping allowance
Shrinkage or Contraction Allowance ( click on Table 1 to view various rate of
contraction of various materials)
All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage
is of two types:
i.
Liquid Shrinkage: it refers to the reduction in volume when the metal changes from
liquid state to solid state at the solidus temperature. To account for this shrinkage;
riser, which feed the liquid metal to the casting, are provided in the mold.
ii.
Solid Shrinkage: it refers to the reduction in volume caused when metal loses
temperature in solid state. To account for this, shrinkage allowance is provided on the
patterns.
The rate of contraction with temperature is dependent on the material. For example steel
contracts to a higher degree compared to aluminum. To compensate the solid shrinkage, a
shrink rule must be used in laying out the measurements for the pattern. A shrink rule for
cast iron is 1/8 inch longer per foot than a standard rule. If a gear blank of 4 inch in
diameter was planned to produce out of cast iron, the shrink rule in measuring it 4 inch
would actually measure 4 -1/24 inch, thus compensating for the shrinkage. The various
rate of contraction of various materials are given in Table 1.
Table 1 : Rate of Contraction of Various Metals
Material
Shrinkage
Dimension
allowance
(inch/ft)
Grey Cast Iron
Up
2
to
feet
2
to
feet 0.125
4
over 4 feet
Cast Steel
Up
2
0.083
to
feet
feet 0.251
2
to
6
Up
4
to
feet
Up
to
to
Over 4 feet
feet 0.155
4
6
over 6 feet
Magnesium
feet 0.191
0.155
over 6 feet
Aluminum
feet 0.105
feet 0.143
0.125
4
feet 0.173
0.155
Draft or Taper Allowance
By draft is meant the taper provided by the pattern maker on all vertical surfaces of the
pattern so that it can be removed from the sand without tearing away the sides of the sand
mold and without excessive rapping by the molder. Figure 3.2 (a) shows a pattern having
no draft allowance being removed from the pattern. In this case, till the pattern is
completely lifted out, its sides will remain in contact with the walls of the mold, thus
tending to break it. Figure 3.2 (b) is an illustration of a pattern having proper draft
allowance. Here, the moment the pattern lifting commences, all of its surfaces are well
away from the sand surface. Thus the pattern can be removed without damaging the mold
cavity.
Figure 3.2 (a) Pattern Having No Draft on Vertical Edges
Figure 3.2 (b) Pattern Having Draft on Vertical Edges
Draft allowance varies with the complexity of the sand job. But in general inner details of
the pattern require higher draft than outer surfaces. The amount of draft depends upon the
length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the
method of molding; and pattern material. Table 2 provides a general guide lines for the
draft allowance.
Table 2 : Draft Allowances of Various Metals
Pattern material
Wood
Metal and plastic
Height of the given Draft angle
Draft angle
surface (inch)
(External surface)
(Internal surface)
1
3.00
3.00
1 to 2
1.50
2.50
2 to 4
1.00
1.50
4 to 8
0.75
1.00
8 to 32
0.50
1.00
1
1.50
3.00
1 to 2
1.00
2.00
2 to 4
0.75
1.00
4 to 8
0.50
1.00
8 to 32
0.50
0.75
Machining or Finish Allowance
The finish and accuracy achieved in sand casting are generally poor and therefore when
the casting is functionally required to be of good surface finish or dimensionally accurate,
it is generally achieved by subsequent machining. Machining or finish allowances are
therefore added in the pattern dimension. The amount of machining allowance to be
provided for is affected by the method of molding and casting used viz. hand molding or
machine molding, sand casting or metal mold casting. The amount of machining
allowance is also affected by the size and shape of the casting; the casting orientation; the
metal; and the degree of accuracy and finish required. The machining allowances
recommended for different metal is given in Table 3.
Table 3 : Machining Allowances of Various Metals
Metal
Cast iron
Cast steel
Non ferrous
Dimension (inch)
Allowance (inch)
Up to 12
0.12
12 to 20
0.20
20 to 40
0.25
Up to 6
0.12
6 to 20
0.25
20 to 40
0.30
Up to 8
0.09
8 to 12
0.12
12 to 40
0.16
Distortion or Camber Allowance
Sometimes castings get distorted, during solidification, due to their typical shape. For
example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract
at the closed end causing the vertical legs to look slightly inclined. This can be prevented
by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that
the casting after distortion will have its sides vertical.
The distortion in casting may occur due to internal stresses. These internal stresses are
caused on account of unequal cooling of different section of the casting and hindered
contraction. Measure taken to prevent the distortion in casting include:
i.
Modification of casting design
ii.
Providing sufficient machining allowance to cover the distortion affect
iii.
Providing suitable allowance on the pattern, called camber or distortion allowance
(inverse reflection)
Figure 3.4: Distortions in Casting
Rapping Allowance
Before the withdrawal from the sand mold, the pattern is rapped all around the vertical
faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the
final casting made, it is desirable that the original pattern dimension should be reduced to
account for this increase. There is no sure way of quantifying this allowance, since it is
highly dependent on the foundry personnel practice involved. It is a negative allowance
and is to be applied only to those dimensions that are parallel to the parting plane.
Core and Core Prints
Castings are often required to have holes, recesses, etc. of various sizes and shapes. These
impressions can be obtained by using cores. So where coring is required, provision
should be made to support the core inside the mold cavity. Core prints are used to serve
this purpose. The core print is an added projection on the pattern and it forms a seat in the
mold on which the sand core rests during pouring of the mold. The core print must be of
adequate size and shape so that it can support the weight of the core during the casting
operation. Depending upon the requirement a core can be placed horizontal, vertical and
can be hanged inside the mold cavity. A typical job, its pattern and the mold cavity with
core and core print is shown in Figure 3.5.
Figure 3.5: A Typical Job, its Pattern and the Mold Cavity
3.6.Types of Patterns
Patterns are of various types, each satisfying certain casting requirements.
1.
Single piece pattern
2.
Split or two piece pattern
3.
Match plate pattern
3.6.1. Single Piece Pattern
The one piece or single pattern is the most inexpensive of all types of patterns. This type
of pattern is used only in cases where the job is very simple and does not create any
withdrawal problems. It is also used for application in very small-scale production or in
prototype development. This type of pattern is expected to be entirely in the drag and one
of the surface is is expected to be flat which is used as the parting plane. A gating system
is made in the mold by cutting sand with the help of sand tools. If no such flat surface
exists, the molding becomes complicated. A typical one-piece pattern is shown in Figure
3.6.
Figure 3.6: A Typical One Piece Pattern
3.6.2. Split or Two Piece Pattern
Split or two piece pattern is most widely used type of pattern for intricate castings. It is
split along the parting surface, the position of which is determined by the shape of the
casting. One half of the pattern is molded in drag and the other half in cope. The two
halves of the pattern must be aligned properly by making use of the dowel pins, which
are fitted, to the cope half of the pattern. These dowel pins match with the precisely made
holes in the drag half of the pattern. A typical split pattern of a cast iron wheel Figure 3.7
(a) is shown in Figure 7 (b).
Figure 3.7 (a): The Details of a Cast Iron Wheel
Figure 3.7 (b): The Split Piece or Two Piece Pattern of a Cast Iron Wheel
3.6.3. Match-Plate Patterns / Cope-And-Drag Pattern
For higher production quantities, match-plate patterns or cope-and-drag patterns are used.
In match-plate patterns the two pieces of the split pattern are attached opposite sides of a
wood or metal plate. Holes in the place allow the top and bottom (cope and drag) sections
of the mold to be aligned accurately. Cope-and-drag patterns me similar to match-plate
patterns except that split pattern halves are attached to separate plates, so that the cope
and drag sections of the mold can be fabricated independently, instead of using the same
tooling for both. Part (b) of the figure includes the gating and riser system in the copeand-drag patterns
(a) match
match-plate pattern, (b) cope and drag pattern
pattern:
3.6.4. Loose piece pattern
It is a pattern with loose pieces, which are necessary to facilitate withdrawal of the
pattern from the mould. A loose piece pattern is shown below. This type of pattern is
used when the contour of the part is such that withdrawal of the pattern from the mould is
not possible. This type of pattern is also used in situations where the casting is having
projections, undercuts, or other configurations that would otherwise hinder the removal
of the pattern. Hence, during moulding the obstructing part of the contour is held as a
loose-piece
piece by the wire. The portion of the pattern liable to cause obstruction in
withdrawal is prepared as a loose part, called loose
loose-pieces,
pieces, which can be attached or
detached as required. After ramming is over, the main pattern is removed and then loose
pieces are withdrawn through the gap generated by the main pattern. Moulding with loose
pieces is a highly skilled and generally expensive job, therefore, should be avoided.
3.6.5. Gated pattern:
For producing small-sized
sized castings, in one mould many cavities may be made. This is
done by making a gated pattern in which number of small patterns, of the desired casting,
are attached to a single runner by means of gates. Generally, gated patterns for eight
small castings is illustrated below.
3.6.6. Sweep Pattern
It is used to sweep the complete casting by means of a plane sweep. These are used for
generating large shapes which are axi
axi-symmetrical
symmetrical or prismatic in nature such as bell
shaped or cylindrical. This greatly reduces the cost of a three dimensional pattern.
pa
It is
suitable for very large castings such as the bells for ornamental purposes used which are
generally cast in pit moulds.
3.6.7. Skeleton Pattern
It is made of strips of wood and is used for building the final pattern by packing sand
around the skeleton. After packing the sand, the desired form is made with the help of a
stickle.. This type of pattern is useful for large castings, required in small qquantities
uantities where
large expense on complete wooden pattern is not justified.
3.6.8. Follow Board Pattern
This type of pattern is adopted for those castings where there are some portions which are
structurally weak and if not supported properly are likely to break under the force of
ramming. Hence the bottom board is modified as a follow board to closely fit the contour
of the weak pattern and thus support it during the ramming of the drag.
Fig. 3.16: Follow Board Pattern
Segmental patterns:-These
These patterns are used for preparing moulds of large circular
castings, avoiding the use of a solid pattern of the exact size.
In principle they work like a sweep, but the difference is that a sweep is given a
continuous revolving motion to generate the desired shape, whereas a segmental pattern
is a portion of the solid pattern itself and the mould is prepared in parts by it. It is
mounted on a central pivot and after preparing the part mould in one position, the
segment is moved to the next position. The operation is repeated till the complete mould
is ready. A typical example is shown in Fig.
3.6.9. Lagged up pattern:
Cylindrical patterns, example, barrels, pipes or columns are built up with lag or stave
construction to ensure proper shape. Longitudinal strips of wood, called lags or staves are
beveled on each side and glued to the wooded pieces called “heads”. Such a construction
gives the maximum amount of strength and permits building close to the finished outline
of the pattern so that there is comparatively little excess stock to be removed to bring it to
the required form.
Lagged-up pattern
3.6.10. Shell Pattern:
A shell pattern is largely used for drainage fittings and pipe work. This type of pattern is
usually made of metal and parted along the center line, the two sections being accurately
dowelled together. The short bends are usually moulded and cast in pairs. The shell
pattern is a hollow construction like shell. The outside shape is used as a pattern to make
the mould, while the inside is used as a core box for making cores.
Shell pattern
3.6.11. Left and Right Hand Patterns:
Some patterns are required to be in pairs, and when their form is such that they cannot be
reversed and have the centers of the hubs, bosses, etc., opposite and in line, then they
must be made right and left hand separately. A few examples where a pair of left and
right hand patterns is required are legs for wood turning lathe, J-hangers for overhead
shafting, legs for garden bench, legs for paddle type sewing machine, brackets for
luggage racks in the railway carriages etc.
A bracket as shown below is a example for left and right hand pattern. The hub and the
foot flangers are fastened with screws are moved from side to side to make the pattern
right and left hand, as shown by dotted lines in the figure. The ribs are also loose. The rib
C is reversed, but right and left hand ribs D are required because of the angle at the lower
edge.
Left and Right hand pattern
3.7.Core boxes
Whenever a hole, recess, undercut or internal cavity is required in a casting, a core, which
is usually made up of a refractory material like sand is inserted at the required location in
the mould cavity before finally closing the mould.
A core, being surrounded on all sides by molten metal, should be able to withstand high
temperature. It should also be adequately supported otherwise due to buoyancy of molten
metal, it will get displaced. When the molten metal around the core solidifies and shrinks,
the core should give way, otherwise the casting may crack (hot tear). Cores, as explained
previously, should be made of oil sand and dried in owens before use.
Cores are made with the help of core boxes. Core boxes are made of wood and have a
cavity cut in them, which is the shape and size of the core. The sand in mixed and filled
in the core boxes. It is then rammed. A core box is made in two halves, each half contains
half impression of core. Sometimes a core may need reinforcements to hold it together.
The reinforcements are in the shape of wire or nails, which can be extracted from the hole
in the casting along with core sand.
Patterns define the external shape of the cast part. If the casting is to have internal
surfaces, a core is required. A core is a full-scale model of the interior surfaces of the
part. It is inserted into the mold cavity prior to pouring, so that the molten metal will flow
and solidify between the mold cavity and the core to form the casting's external and
internal surfaces. The core is usually made of sand, compacted into the desired shape. As
with the pattern, the actual size of the Gore must include allowances for shrinkage and
machining. Depending on the geometry of the part, the core may or may not require
supports to fig 3.17. Patterns and Cores hold it in position in the mold cavity during
pouring. These supports, called chaplets, are made of a metal with a higher melting
temperature than the casting metal. For example, steel chaplets would be used for
castings. On pouring and solidification, the chaplets become bonded in to the casting. A
possible arrangement of a core in a mold using chaplets is sketched in figure 3.17 the
portion of the chaplet protruding from the casting is subsequently cut off.
Figure 3.17 (a) core held in place in the mold cavity by chaplets, (b) possible chaplet
design, and (c) casting with internal cavity.
3.8.Summary
In this unit we studied Pattern Materials, Pattern Making Tools, Pattern Allowances,
Types of Patterns, Solid or Single Piece Pattern, Split Pattern, Match Plate Pattern, Cope
and Drag Pattern, Loose Piece Pattern, Gated Pattern, Sweep Pattern, Skeleton Pattern,
Shell Pattern, Segmental Pattern, Follow Board Pattern, Lagged-up Pattern, Left and
Right hand Pattern, Core Boxes, Colour coding for Pattern and Core Boxes.
3.9.Keywords
− Split Pattern
− Match Plate Pattern
− Cope and Drag Pattern
− Loose Piece Pattern
− Gated Pattern
− Sweep Pattern
− Skeleton Pattern
− Segmental Pattern
− Follow Board Pattern
− Lagged-up Pattern
− Shell Pattern
3.10.
Exercise
1. Explain the functions of the pattern.
2. What is pattern material?
3. What are the different types of patterns?
Unit 4
Casting Processes
Structure
4.1.Introduction
4.2.Objectives
4.3.Casting Terms
4.4.Permanent Mould Casting
4.5.Slush Casting
4.6.Die Casting
4.7.Centrifugal Casting
4.8.Investment Casting
4.9.Shell Moulding Process
4.10.
Continuous Casting
4.11.
Defects in Casting
4.12.
Design of Castings
4.13.
Cleaning of Castings
4.14.
Inspection of Castings
4.15.
Summary
4.16.
Keywords
4.17.
Exercise
4.1.Introduction
Virtually nothing moves, turns, rolls, or flies without the benefit of cast metal products.
The metal casting industry plays a key role in all the major sectors of our economy. There
are castings in locomotives, cars trucks, aircraft, office buildings, factories, schools, and
homes. Figure1.1 some metal cast parts.
Metal Casting is one of the oldest materials shaping methods known. Casting means
pouring molten metal into a mold with a cavity of the shape to be made, and allowing it
to solidify. When solidified, the desired metal object is taken out from the mold either by
breaking the mold or taking the mold apart. The solidified object is called the casting. By
this process, intricate parts can be given strength and rigidity frequently not obtainable by
any other manufacturing process. The mold, into which the metal is poured, is made of
some heat resisting material. Sand is most often used as it resists the high temperature of
the molten metal. Permanent molds of metal can also be used to cast products.
Figure 1.1: Metal Cast parts
4.2.Objectives
After studying this unit we are able to understand
− Casting Terms
− Permanent Mould Casting
− Slush Casting
− Die Casting
− Centrifugal Casting
− Investment Casting
− Shell Moulding Process
− Continuous Casting
− Defects in Casting
− Design of Castings
− Cleaning of Castings
− Inspection of Castings
Advantages
The metal casting process is extensively used in manufacturing because of its many
advantages.
1. Molten material can flow into very small sections so that intricate shapes can be made
by this process. As a result, many other operations, such as machining, forging, and
welding, can be minimized or eliminated.
2. It is possible to cast practically any material that is ferrous or non-ferrous.
3. As the metal can be placed exactly where it is required, large saving in weight can be
achieved.
4. The necessary tools required for casting molds are very simple and inexpensive. As a
result, for production of a small lot, it is the ideal process.
5. There are certain parts made from metals and alloys that can only be processed this
way.
6. Size and weight of the product is not a limitation for the casting process.
Limitations
1. Dimensional accuracy and surface finish of the castings made by sand casting
processes are a limitation to this technique. Many new casting processes have been
developed which can take into consideration the aspects of dimensional accuracy and
surface finish. Some of these processes are die casting process, investment casting
process, vacuum-sealed molding process, and shell molding process.
2. The metal casting process is a labor intensive process
4.3.Casting Terms
1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is
formed. Depending upon the position of the flask in the molding structure, it is
referred to by various names such as drag – lower molding flask, cope – upper
molding flask, cheek – intermediate molding flask used in three piece molding.
2. Pattern: It is the replica of the final object to be made. The mold cavity is made with
the help of pattern.
3. Parting line: This is the dividing line between the two molding flasks that makes up
the mold.
4. Molding sand: Sand, which binds strongly without losing its permeability to air or
gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions.
5. Facing sand: The small amount of carbonaceous material sprinkled on the inner
surface of the mold cavity to give a better surface finish to the castings.
6. Core: A separate part of the mold, made of sand and generally baked, which is used to
create openings and various shaped cavities in the castings.
7. Pouring basin: A small funnel shaped cavity at the top of the mold into which the
molten metal is poured.
8. Sprue: The passage through which the molten metal, from the pouring basin, reaches
the mold cavity. In many cases it controls the flow of metal into the mold.
9. Runner: The channel through which the molten metal is carried from the sprue to the
gate.
10. Gate: A channel through which the molten metal enters the mold cavity.
11. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of
its own weight and overcome the metallostatic force.
12. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks
and solidifies. Also known as “feed head”.
13. Vent: Small opening in the mold to facilitate escape of air and gases.
Figure 1 : Mold Section showing some casting terms
4.4.Permanent-Mold Casting
The economic disadvantage of any of the expendable mold processes is that a new mold
is required for every casting. In permanent-mold casting, the mold is reused many times.
In this section, we treat permanent-mold casting as the basic process in the group of
casting processes that all use reusable metal molds. Other members of the group include
die casting and centrifugal casting.
The Basic Permanent-Mold Process
Permanent-mold casting uses a metal mold constructed of two sections that are designed
for easy, precise opening and closing. These molds are commonly made of steel or cast
iron. The cavity, with gating system included, is machined into the two halves to provide
accurate dimensions and good surface finish. Metals commonly cast in permanent molds
include aluminum, magnesium, copper-base alloys, and cast iron. However, cast iron
requires a high pouring temperature, 1250°C to 1500°C (230PF-Z700"F), which takes a
heavy toll on mold life. The very high pouring temperatures of steel make permanent
molds unsuitable for this metal, unless the mold is made of refractory material.
Cores can be used in permanent molds to form interior surfaces in the cast product. The
cores can be made of metal, but either their shape must allow for removal film the casting
or they must be mechanically collapsible to permit removal. If withdrawal of a metal core
would be difficult or impossible, sand wires can be used, in which case the casting
process is often referred to as semi permanent-mold casting. Steps in the basic permanent
mold casting process are described in Figure 11.20. In preparation for casting, the mold is
first preheated and one or more coatings are sprayed on the cavity. Preheating facilitates
metal flow through the gating system and into the cavity. The coatings aid heat
dissipation and lubricate the mold surfaces for easier separation of the cast product. After
pouring as soon as the metal solidifies, the mold is opened and the casting is removed.
Unlike expendable molds, permanent molds do not collapse, so the mold must be opened
before appreciable cooling contraction occurs in order to prevent cracks from developing
in the casting. Advantages of permanent-mold casting include good surface finish and
close dimensional control, as previously indicated. In addition, more rapid solidification
is caused by the metal mold results in a finer grain structure, so stronger castings are
produced. The process is generally limited to metals of tower melting points. Other
Limitations include simple part geometries compared to sand casting (because of the need
to open the mold), and the expense of the mold. Because mold cosr is substantial, the
process is best suited to high-volume production and can be automated accordingly.
Typical parts include automotive pistons, pump bodies, and certain castings for aircraft
and missile.
FIGURE 1 -1 Steps in permanent-mold casting: (1) mold is preheated and coated: (2)
cores (if used) are inserted, and mold is closed; (3) molten metal is poured into the mold;
and (4) mold is cleaned. Finished part is shown in (5).
Variations of Permanent-Mold Casting
Several casting processes are quite similar to the basic permanent-mold method. These
include slush casting, low-pressure casting, and vacuum permanent-mold casting.
4.5.Semi-permanent Mould Casting
Semi-permanent mold is a casting process - producing Aluminum alloy castings - using
re-usable metal molds and sand cores to form internal passages within the casting. Molds
are typically arranged in two halves - the sand cores being put into place before the two
halves are placed together. The molten metal flows into the mold cavity and surrounds
the sand core while filling the mold cavity. When the casting is removed from the mold
the sand core is removed from the casting leaving an internal passage in the casting.
The re-usable metal molds are used time and again, but the sand cores have to be
replaced each time the product is cast, hence the term semi-permanent molding.
Semi-permanent molding affords a very high precision quality to the casting at a reduced
price compared to the sand casting processes.
4.6.Slush Casting
Slush casting is a permanent mold process in which a hollow casting is formed, by
inverting the mold after partial freezing at the surface to drain out the liquid metal in the
center. Solidification begins at the mold walls because they are relatively cool, and it
progresses over time toward the middle of the casting (Section 10.3.1). Thickness of the
shell is controlled by the length of time allowed before draining. Slush casting is used to
make statues, lamp pedestals, and toys out of low-melting-point metals such as lead, zinc,
and tin. In these items, the exterior appearance is important, but the strength and interior
geometry of the casting are minor considerations.
Low-Pressure Casting In the basic permanent mold casting process and in slush casting,
the flow of metal into the mold cavity is caused by gravity. In low-pressure casting, the
liquid metal is forced into the cavity under low pressure-approximately 0.1 MPa (15
lb/in2)-from beneath so that the flow is upward, as illustrated in Figure 1.1. The
advantage of this approach over traditional pouring is that clean molten metal from the
center of the ladle is introduced into the mold, rather than metal that has been exposed to
air. Gas porosity and oxidation defects are thereby minimized, and mechanical properties
are improved.
Vacuum Permanent-Mold Casting Vacuum permanent mold casting (not to be
confused with vacuum molding) is a variation of low-pressure casting in which a vacuum
is used to draw the molten metal into the mold cavity. The general configuration of the
vacuum permanent-mold casting process is similar to the low-pressure casting operation.
The difference is that reduced air pressure from the vacuum in the mold is used to draw
the liquid metal into the cavity, rather than forcing it by positive air pressure from below.
There are several benefits of the vacuum technique relative to low-pressure casting: air
porosity and related defects are reduced, and greater strength is given to the cast product.
FIGURE 1.2 Low-pressure casting. 'The diagram shows how air pressure is used to force
the molten metal in the ladle upward into the mold cavity. Pressure is maintained until the
casting has solidified.
4.7.Die Casting
Die casting is a permanent-mold casting process in which the molten metal is injected
into the mold cavity under high pressure. Typical pressures are 7 to 350 MPa (100050,000 Ibiin2). The pressure is maintained during solidification, after which the mold is
opened and the part is removed. Molds in this casting operation are called dies; hence the
name, die casting. The use of high pressure to force the metal into the die cavity is the
most notable feature that distinguishes this process from others in the permanent mold
category.
Die-casting operations are carried out in special die-casting machines. Modern diecasting machines are designed to hold and accurately close the two halves of the mold,
and keep them closed while the liquid metal is forced into the cavity. The general
configuration is shown in Figure 1.3. There are two main types of die-casting machines:
(1) hot-chamber and (2) cold-chamber, differentiated by how the molten metal is injected
into the cavity.
Figure 1.3 General configuration of a (cold-chamber) die-casting machine.
4.8.Centrifugal Casting
Centrifugal casting refers to several casting methods in which the mold is rotated at high
speed so that centrifugal force distributes the molten metal to the outer regions of the die
cavity. The group includes (1) true centrifugal casting, (2) semi centrifugal casting, and
(3) centrifuge casting.
FIGURE1.4: Setup for true centrifugal casting.
True Centrifugal casting in true centrifugal casting, molten metal is poured into a rotating
mold to produce a tubular part. Examples of parts made by this process include pipes,
tubes, bushings, and rings. One possible setup is illustrated in Figure 1.4. Molten metal is
poured into a horizontal rotating mold at one end, In some operations, mold rotation
commences after pouring has occurred rather than beforehand. The high-speed relation
results in centrifugal forces that cause the metal to take the shape of the mold cavity.
Thus, the outside shape of the casting can be round, octagonal, hexagonal, and so on.
However, the inside shape of the casting is (theoretically) perfectly round, due to the
radially symmetric forces at work. Orientation of the axis of mold rotation can be either
horizontal or vertical, the former being more common. Let us consider how fast the mold
must rotate in horizontal centrifugal costing for the process to work successfully.
Centrifugal force is defined by this physics equation:
11.2
where F F= force, N (lb); rn = mass, kg (lbm); v = velocity, m/s; and Rt= inside radius
of the mold, m. The force of gravity is its weight W = mg, where W is given in kg (lb),
and g = acceleration of gravity, 9.8 m/s2. The so-called G-factor GF is the ratio of
centrifugal force divided by weight:
11.3
Velocity v can be expressed as 2πRN/6O = πRN/30, where N = rotational speed.
Rev/min.
Substituting this expression into Eq. (11.3), we obtain
11.4
Rearranging this to solve for rotational speed N, and using diameter D rather than radius
in the resulting equation, we have
11.5
where D = inside diameter of the mold, rn (ft). If the G-factor is too low in centrifugal
casting, the liquid metal will not remain forced against the mold wall during the upper
half of the circular path but will "rain" inside the cavity. Slipping occurs between the
molten metal and the mold wall, which means that the rotational speed of the metal is
less than that of the mold. On an empirical basis, values of GF = 60 to 80 are found to
be appropriate for horizontal centrifuga1 casting, although this depends to some extent
on the metal being cast.
4.9.Investment Casting
The root of the investment casting process, the cire per due or “lost wax” method dates
back to at least the fourth millennium B.C. The artists and sculptors of ancient Egypt and
Mesopotamia used the rudiments of the investment casting process to create intricately
detailed jewelry, pectorals and idols. The investment casting process also called lost wax
process begins with the production of wax replicas or patterns of the desired shape of the
castings. A pattern is needed for every casting to be produced. The patterns are prepared
by injecting wax or polystyrene in a metal dies. A number of patterns are attached to a
central wax sprue to form a assembly. The mold is prepared by surrounding the pattern
with refractory slurry that can set at room temperature. The mold is then heated so that
pattern melts and flows out, leaving a clean cavity behind. The mould is further hardened
by heating and the molten metal is poured while it is still hot. When the casting is
solidified, the mold is broken and the casting ttaken out.
The basic steps of the investment casting process are :
1.
Production of heat-disposable
disposable wax, plastic, or polystyrene patterns
2.
Assembly of these patterns onto a gating system
3.
“Investing,” or covering the pattern assembly with refractory slurry
4.
Melting
ng the pattern assembly to remove the pattern material
5.
Firing the mold to remove the last traces of the pattern material
6.
Pouring
7.
Knockout, cutoff and finishing.
4.10.
The Basic Steps of the Investment Casting Process
Advantages
•
Formation of hollow interiors in cylinders without cores
•
Less material required for gate
•
Fine grained structure at the outer surface of the casting free of gas and shrinkage
cavities and porosity
Disadvantages
•
More segregation of alloy component during pouring under the forces of rotat
rotation
•
Contamination of internal surface of castings with non
non-metallic
metallic inclusions
•
Inaccurate internal diameter
4.11.
Shell Molding Process
It is a process in which, the sand mixed with a thermosetting resin is allowed to come in
contact with a heated pattern plate (200 oC), this causes a skin (Shell) of about 3.5 mm of
sand/plastic mixture to adhere to the pattern.. Then the shell is removed from the pattern.
The cope and drag shells are kept in a flask with necessary backup material and the
molten metal is poured into the mold.
This process can produce complex parts with good surface finish 1.25 µm to 3.75 µm,
and dimensional tolerance of 0.5 %. A good surface finish and good size tolerance reduce
the need for machining. The process overall is quite cost effective due to reduced
machining and cleanup costs. The materials that can be used with this process are cast
irons, and aluminum and copper alloys.
Molding Sand in Shell Molding Process
The molding sand is a mixture of fine grained quartz sand and powdered bake
bakelite. There
are two methods of coating the sand grains with bakelite. First method is Cold coating
method and another one is the hot method of coating.
In the method of cold coating, quartz sand is poured into the mixer and then the solution
of powdered bakelite
kelite in acetone and ethyl aldehyde are added. The typical mixture is
92% quartz sand, 5% bakelite, 3% ethyl aldehyde. During mixing of the ingredients, the
resin envelops the sand grains and the solvent evaporates, leaving a thin film that
uniformly coatss the surface of sand grains, thereby imparting fluidity to the sand
mixtures.
In the method of hot coating, the mixture is heated to 150
150-180
180 o C prior to loading the
sand. In the course of sand mixing, the soluble phenol formaldehyde resin is added. The
mixer
ixer is allowed to cool up to 80 – 90 o C. This method gives better properties to the
mixtures than cold method.
4.12.
Continuous casting
Continuous casting,, also called strand casting, is the process whereby molten metal is
solidified into a "semifinished" billet, bloom, or slab for subsequent rolling in the
finishing mills.
Molten metal (known as hot metal in industry) is tapped into the ladle from furnaces.
After undergoing any ladle treatments, such as alloying and degassing, and arriving at the
correct temperature, the ladle is transported to the top of the casting machine. Usually,
the ladle sits in a slot on a rotating turret at the casting machine; one ladle is 'on cast'
(feeding the casting machine) while the other is made ready, and is switched to the
casting position once the first ladle is empty.
From the ladle, the hot metal is transferred via a refractory shroud (pipe) to a holding
bath called a tundish. The tundish allows a reservoir of metal to feed the casting machine
while ladles are switched, thus acting as a buffer of hot metal, as well as smoothing out
flow, regulating metal feed to the molds and cleaning the metal (see below).
Metal is drained from the tundish through another shroud into the top of an open-base
copper mold. The depth of the mold can range from 0.5 to 2 metres (20 to 79 in),
depending on the casting speed and section size. The mold is water-cooled to solidify the
hot metal directly in contact with it; this is the primary cooling process. It also oscillates
vertically (or in a near vertical curved path) to prevent the metal sticking to the mold
walls. A lubricant can also be added to the metal in the mold to prevent sticking, and to
trap any slag particles—including oxide particles or scale—that may still be present in
the metal and bring them to the top of the pool to form a floating layer of slag. Often, the
shroud is set so the hot metal exits it below the surface of the slag layer in the mold and is
thus called a submerged entry nozzle (SEN). In some cases, shrouds may not be used
between tundish and mold; in this case, interchangeable metering nozzles in the base of
the tundish direct the metal into the moulds. Some continuous casting layouts feed
several molds from the same tundish.
In the mold, a thin shell of metal next to the mold walls solidifies before the middle
section, now called a strand, exits the base of the mold into a spray-chamber; the bulk of
metal within the walls of the strand is still molten. The strand is immediately supported
by closely spaced, water cooled rollers; these act to support the walls of the strand against
the ferrostatic pressure (compare hydrostatic pressure) of the still-solidifying liquid
within the strand. To increase the rate of solidification, the strand is also sprayed with
large amounts of water as it passes through the spray-chamber; this is the secondary
cooling process. Final solidification of the strand may take place after the strand has
exited the spray-chamber.
It is here that the design of continuous casting machines may vary. This describes a
'curved apron' casting machine; vertical configurations are also used. In a curved apron
casting machine, the strand exits the mold vertically (or on a near vertical curved path)
and as it travels through the spray-chamber, the rollers gradually curve the strand towards
the horizontal. In a vertical casting machine, the strand stays vertical as it passes through
the spray-chamber. Molds in a curved apron casting machine can be straight or curved,
depending on the basic design of the machine.
In a true "Horizontal Casting Machine", the mold axis is horizontal and the flow of steel
is horizontal from liquid to thin shell to solid (no bending). In this type of machine, either
strand oscillation or mold oscillation is used to prevent sticking in the mold.
After exiting the spray-chamber, the strand passes through straightening rolls (if cast on
other than a vertical machine) and withdrawal rolls. There may be a hot rolling stand after
withdrawal, in order to take advantage of the metal's hot condition to pre-shape the final
strand. Finally, the strand is cut into predetermined lengths by mechanical shears or by
travelling oxyacetylene torches, is marked for identification and either taken to a
stockpile or the next forming process.
In many cases the strand may continue through additional rollers and other mechanisms
which might flatten, roll or extrude the metal into its final shape.
4.13.
Defects in casting
There are numerous opportunities for things to go wrong in a casting operation, resulting
in quality
uality defects in the cast product. In this section, we compile a list of the common
defects that occur in casting, and we indicate the inspection procedures to detect them.
Casting Defects some
ome defects are common to any and all casting processes, these defects
are illustrated in Figure 1.5 and briefly described in the following:
(a) Misruns, which are castings that solidify before completely filling the mold cavity.
Typica1-causes include (1) fluidity of the molten metal is insufficient, (2) pourin
pouring
temperature is too low, (3) pouring is done too slowly, and or (4) cross section of the
mold cavity is too thin.
FIGURE 1.5 Some common defects in castings: (a) misrun, (b) cold shut, (c) cold shot
(dl shrinkage cavity, (e) micro porosity, and (f) hot tearing
(b) Cold Shuts, which occur when two portions of the metal Row together' but there is a
lack of fusion between them due ea premature freezing. Its causes are similar to those of
a misrun.
(c) Cold shots, which result from splattering during pouring, causing the formation of
solid globules of metal that become entrapped in the casting. Pouring procedures and
gating system designs that avoid splattering can prevent this defect.
(d) Shrinkage cavity is a depression in the surface or an internal void in the casting,
caused by solidification shrinkage that restricts the amount of molten metal available in
the last region to freeze. 3t often occurs near the top of the casting, in which case is
referred lo as a "pipe". The problem can often be solved by proper riser design.
(e) Micro porosity consists of a network of small voids distributed throughout the casting
caused by localized solidification shrinkage of the final molten metal in the dendritic
structure. The defect is usually associated with alloys, because of the protracted manner
in which freezing occurs in these metals.
(f) Hot tearing, also called hot cracking, occurs when the casting is restrained from
contraction by an unyielding mold during the ha1 stages of solidification or early stages
of cooling after solidification. The defect is manifested as a separation of she metal
(hence, the terms tearing and cracking) at a point of high tensile stress caused by the
metal's inability to shrink naturally. In sand casting and other expendable-mold processes,
it is prevented by compounding the mold to be collapsible. In permanent-mold processes,
hot tearing is reduced by removing the part from the mold immediately after
solidification.
Some defects are related to the use of sand maids, and therefore they occur only in sand
castings. To a lesser degree, other expendable mold processes are also susceptible to
these problems. Defects found primarily in sand castings are shown in Figure 1.6 and
described here:
FIGURE 1 -6 Common defects in sand castings: (a) sand blow, (b) pin holes, (c)
sand wash, (d) scabs, (e) penetration, (f) mold shift, (g) core shift, and (h) mold crack.
(a) Sand blow is a defect consisting of a balloon-shaped gas cavity caused by release of
mold gases during pouring. It occurs at or below the casting surface near the top of
the casting Low permeability, poor venting, and high moisture content of the sand
mold are the usual causes.
(b) Pinholes, also caused by release of gases during pouring, consist of many small gas
cavities formed at or slightly below the surface of the casting.
(c) Sand wash, which is an irregularity in the surface of the casting that results from
erosion of the sand mold during pouring, and the contour of the erosion is formed in
the surface of the final cast part.
(d) Scabs are rough areas on the surface of the casting due to encrustations of sand and
meta t. It is caused by portions of the mold surface flaking off during solidification
and becoming imbedded in the casting surface.
(e) Penetration refers to a surface defect that occurs when the fluidity of the liquid metal
is high, and it penetrates into the sand mold or sand core. Upon freezing, the casting
surface consists of a mixture of sand grains and metal. Harder packing of he sand
mold helps to alleviate this condition.
(f) Mold shift refers to a defect caused by a sidewise displacement of the mold cope
relative to the drag, the result of which is a step in the most product at the parting line.
(g) Core shift is similar to mold shift, but it is the core that is displaced, and the
displacement is usually vertical. Core shift and mold shift are caused by buoyancy of
the molten metal.
(h) Mold ma& occurs when mold strength is insufficient, and a crack develops, into
which liquid metal can seep to form a "fin" on the final casting.
Inspection Methods Foundry inspection procedures include
(1) Visual inspection to detect obvious defects such as misruns, cold shuts, and severe
surface flaws;
(2) Dimensional measurements to ensure that tolerances have been met; and
(3) Metallurgical, chemical, physical, and other tests concerned with the inherent quality
of the cast metal. Tests in category (3) include:
(a) pressure testing-to locate leaks in the casting;
(b) radiographic methods, magnetic particle tests, the use of fluorescent penetrants, and
supersonic testing-to detect either surface or internal defects in the casting; and
(c) mechanical testing to determine properties such as tensile strength and hardness. If
defects are discovered but are not too serious, it is often possible to save the casting by
welding, grinding, or other salvage methods to which the customer has agreed.
4.14.
Product Design
If casting is selected by the product designer as the primary manufacturing process for a
particular component, then certain guidelines should be followed to facilitate production
of the part and avoid in any of the defects enumerated. Some of the important guidelines
and considerations for casting are presented next.
Geometric simplicity: Although casting is a process that can be used to produce complex
pars geometries, simplifying the part design will improve its capability, Avoiding
unnecessary complexities simplifies mold making, reduces the need for cores, and
improves the strength of the casting.
Corners: Sharp corners and angles should be avoided, because they are sources of stress
concentrations and may cause hot tearing and cracks in the casting. Generous fillets
should be designed on inside corners, and sharp edges should be blended.
Section thickness: Section thickness should be uniform in order to avoid shrinkage
cavities. Thicker sections create hot spot in the casting, because greater volume requires
more time for solidification and cooling. These are likely locations of shrinkage cavities.
Figure 1.6 illustrates the problem and offers some possible solutions.
Draft: Part sections that project into the mold should have a draft or taper, as defined in
Figure 1.7. In expendable mold casting the purpose of this draft is to facilitate removal of
the pattern from the mold. In permanent mold casting, its purpose is to aid in removal of
the part from the mold. Similar tapers should be allowed if solid cores are used in the
casting process. The required draft need only be about lo for sand casting and 2" to 3" for
permanent mold processes.
FIGURE 1 -6 (a) Thick section at intersection can result in a shrinkage cavity. Remedies
include (b) redesign to reduce thickness and (c) use of a core
Figure 1.7: Design change to eliminate the need for using a core: (a) original design and
(b) redesign.
Use of cores Minor changes in part design can reduce the need for coring, as shown in
Figure 1.7.
Dimension of tolerances. There are significant differences in the dimensional accuracies
that can be achieved in castings, depending on which process is used. Table 1.2 provides
a compilation of typical part tolerances for various casting processes and metals.
Surface finish: Typical surface roughness achieved in sand casting is around 6 fim (250
p-in). Similarly poor finishes are obtained in shell molding, while plaster-mold and
investment casting produce much better roughness values: 0.75 pm (31) pin). Among the
permanent mold processes, die casting is noted for good surface finishes at around 1 pm
(40 p-in).
Machining allowance: Tolerances achievable in many casting processes are insufficient
to meet functional needs in many applications. Sand casting is the most prominent
example of this deficiency. 1n these cases, portions of the casting must be machined to
the required dimensions. Almost all sand castings must be machined to some extent in
order for the part to be made functional. Therefore, additional material, called the
machining allowance is left on the casting for machining those surfaces where necessary.
Typical machining allowances for sand castings range between 1.5 mm and 3 mm (1/16
in and 1/4 in).
TABLE 1.2 Typical dimensional tolerances for various casting processes and metals.
4.15.
Cleaning of Casting
Generally, the cleaning of casting refers to all operations related to the removal of
adhering sand, gates, risers and other metal not a part of the casting. The cleaning
operations may also include a certain amount of metal finishing or machining for
obtaining the required casting dimensions.
The various cleaning operations usually performed on a casting are enumerated and
discussed below:
1. Rough cleaning
2. Surface cleaning
3. Trimming
4. Finishing
1. Rough cleaning: Rough cleaning includes the removal of gates of risers. The
following points are worth-noting
•
In case of a ductile material casting, rough cleaning may be done with
mechanical cut-off machines (using abrasive cut-off wheels, band saws
and metal shears).
•
The gating system of a brittle material casting may be broken off by
impact when the castings are dumped and vibrated in shake-out or knockout devices.
•
In case of steel castings, very large risers and sprues may be removed by
cutting torches.
•
In case of risers being large and cast of oxidation-resisting alloys, powder
cutting (in which a stream of iron powder in introduced into the oxygen
torch flame) is employed.
2. Surface cleaning: Surface cleaning includes cleaning of interior and exterior
surfaces when sand, scale and other adhering materials are involved. This type of
cleaning involves the following procedures:
a. Tumbling: This operation is carried out with a barrel-like machine called
tumbling mill, which removes sand, scale and some fins and wires.
b. Blasting: The sand blasting is performed by using coarse sand as abrasive
and air as the carrying medium. The grit or sand blasting is carried out by
throwing the metallic particles by centrifugal force from a rapidly rotating
wheel
c. Other surface cleaning methods:
The following methods aid in surface cleaning:
− Wire brushing;
− Buffing
− Pickling
− Various polishing procedures
3. Trimming: Trimming involves the removal of fins, gate and riser pads, chaplets,
wires and other similar unwanted appendages to the casting which are not a part
of its final dimensions.
It involves the following procedures:
i.
Chipping: It is used to remove pins, gates and riser pads, wires
etc. It may be carried out by hammer and chisel or by
pneumatic chipping hammers.
ii.
Grinding: It is employed to remove excess metal and is carried
out, through portable grinders, stand grinders and swing-frame
grinders.
4. Finishing: It is the later stage of cleaning. In certain cases cleaning is complete
after trimming operations, but others may required additional surface finishing,
eg., machining, polishing, buffing etc.
Note: The complete process of cleaning of castings, involving the removal of the cores,
gates and risers, cleaning of the casting surface and chipping of any of the unnecessary
projections on the surfaces is known as Fettling.
4.16.
Inspection of Castings
In order to determine the presence of any defects (not readily visible) it becomes
necessary to inspect the casting. Following methods are employed to inspect the casting.
1. Destructive inspection method: In this type of inspection the casting sample is
destroyed during inspection. This method is used to test mechanical properties,
eg., tensile strength, hardness etc. These tests are performed on the test bars or
pieces cut from the casting sample.
2. Non-destructive inspection method: Following are the various methods of nondestructive inspection:
a. Visual inspection: The main aim of this type of inspection is to ensure that
the outward appearance of the casting looks good. Through this inspection
the defects like cracks, tears, run outs, swells etc. may be detected.
b. Dimensional inspection: The dimensional inspection may be carried out
by surface plates, height and depth gauges and plug gauges etc. Through
this inspection it can be ascertained whether certain details are within
tolerances or not.
c. Pressure testing: It is employed to locate leaks in a casting or to check the
overall strength of a casting in resistance to bursting under hydraulic
pressure. It is carried out on tubes and pipes.
d. Radiographic inspection: This type of inspection is employed to inspect
internal defects of a casting, by the use of X-ray or gamma ray technique.
e. Magnetic particle inspection: This inspection method is employed on
magnetic ferrous castings for detecting invisible surface or slightly subsurface defects.
f. Fluorescent penetrant:
•
This type of inspection is employed to find minute pores and cracks on the surface of
castings that may be missed even under magnification.
•
In this method a fluorescent penetrating oil mixed with whiting powder is applied to
the casting surface by dipping, spraying or brushing. The cracks or other defects
become visible after the surface has been wiped dry (the oil creeping out of cracks).
g.
Eddy current inspection: In this method the materials of the casting need
not be ferromagnetic. The test includes a probe which is supplied with a
high frequency current. It induces an electric field in the casting. The field
changes in the presence of surface or near surface defects. These changes
show up on instruments.
4.17.
Summary
In this unit we have studied Permanent Mould Casting, Slush Casting, Die Casting,
Centrifugal Casting, Investment Casting, Shell Moulding Process, Continuous Casting,
Defects in Casting.
4.18.
Keywords
− Slush Casting
− Die Casting
− Centrifugal Casting
− Investment Casting
− Shell Moulding Process
− Continuous Casting
4.19.
Exercise
1. What is casting
2. What are the casting terms?
3. How to do permanent mould casting?
4. Explain different types of casting.
5. Explain the defects of casting.
6. Explain the Cleaning of Castings
7. Explain the Inspection of Castings
Unit 1
WELDING
Structure
1.1.Introduction
1.2.Objectives
1.3.The Weld Joint
1.4.Types of Joints
1.5.Types of Welds
1.6.Welding Operation
1.7.Welding Positions
1.8.Forge Welding
1.9.Resistance Welding
1.10.
Butt Welding Process
1.11.
Arc Welding
1.12.
Electroslag Welding
1.13.
Solid-State Welding
1.14.
Oxyfuel Gas Welding
1.15.
Summary
1.16.
Keywords
1.17.
Exercise
1.1.Introduction
Welding is a materials joining process in which two or more parts are coalesced at their
contacting surfaces by a suitable application of heat andior pressure. Many welding
processes are accomplished by heat alone, with no pressure applied; others by a
combination of heat and pressure; and still others by pressure alone, with no external heat
supplied. In some welding processes a filler material is added to facilitate coalescence.
The assemblage of parts that are joined by welding is called a weldment. Welding is most
commonly associated with metal parts, but the process is also used for joining plastics.
Our discussion of welding will focus on metals.
Welding is a relatively new process. Its commercial and technological importance derives
from the following:
•
Welding provides a permanent joint. The welded parts become a single entity.
•
The welded joint can be stronger than the parent materials if a filler metal is used that
has strength properties superior to those of the parents, and if proper welding
techniques are used.
•
Welding is usually the most economical way to join components in terms of material
usage and fabrication costs. Alternative mechanical methods of assembly require
more complex shape alterations (e.g., drilling of holes) and addition of fasteners (e.g.,
rivets or bolts). The resulting mechanical assembly is usually heavier than a
corresponding weldment.
•
Welding is not restricted to the factory environment. It can be accomplished "in the
field."
•
Although welding has the advantages indicated above, it also has certain limitations
and drawbacks (or potential drawbacks):
•
Most welding operations are performed manually and are expensive in terms of labor
cost. Many welding operations are considered "skilled trades," and the labor to
perform these operations may he scarce.
•
Most welding processes are inherently dangerous because they involve the use of
high energy.
•
Since welding accomplishes a permanent bond between the components, it does not
allow for convenient disassembly. If the product must occasionally he disassembled
(e.g., for repair or maintenance), then welding should not he used as the assembly
method.
•
The welded joint can suffer from certain quality defects that are difficult lo detect the
defects can reduce the strength of the joint.
1.2.Objectives
After studying this unit we are able to understand
− Weld Joint
− Welding Operation
− Welding Positions
− Forge Welding:
− Resistance Welding
− Arc Welding
− Solid-State Welding
− Oxyfuel Gas Welding
1.3.The Weld Joint
Welding produces a solid connection between two pieces, called a weld joint. A weld
joint is the junction of the edges or surfaces of parts that have been joined by welding.
This section covers two classifications related to weld joints: (1) types of joints and (2)
the types of welds used to join the pieces that form the joints.
1.4.Types of Joints
There are five basic types of joints for bringing two parts together for joining. The five
joint types are not limited to welding; they apply to other joining and fastening
techniques as well. With reference to Figure 1.1, the five joint types can be defined as
follows:
(a) Butt joint. In this joint type, the parts lie in the same plane and are joined at their
edges.
(b) Comer joint. The parts in a corner joint form a right angle and are joined at the
corner of the angle.
(c) Lap joint. This joint type consists of two overlapping parts.
(d) Tee joint. In the tee joint, one part is perpendicular to the other in the approximate
shape of the letter "T."
(e) Edge joint. The parts in an edge joint are parallel with at least one of their edges in
common, and the joint is made at the common edge(s).
FIGURE 1.1 Five basic types of joints: (a) butt, (b) corner, (c) lap, (d) tee, and (e) edge
1.5.Types of Welds
Each of the preceding joints can be made by welding. It is appropriate to distinguish
between the joint type and the way in which it is welded-the weld type. Differences
among weld types are in geometry (joint type) and welding process.
A filler weld is used to fill in the edges of plates created by corner, lap, and tee joints, as
in Figure 1.2. Filler metal is used to provide a cross section approximately the shape of a
right triangle. It is the most common weld type in arc and oxy fuel welding because it
requires minimum edge preparation-the basic square edges of the parts are used. Fillet
welds can be single or double (i.e., welded on one side or both) and can be continuous or
intermittent (i.e., welded along the entire length of the joint or with un-welded spaces
along the length).
Figure 1.2 various forms of fillet welds: (a) - inside single fillet corner joint; (b) outside
single fillet corner joint; (c) double fillet lap joint; and (d) double fillet tee joint. Dashed
lines show the original part edges.
Groove welds usually require that the edges of the parts be shaped into a groove to
facilitate weld penetration. The grooved shapes include square, bevel, V, U, and J, in
single or double sides, as shown in Figure 1.3. Filler metal is used to fill in the joint,
usually by arc or oxy fuel welding. Preparation of the part edges beyond the basic square
edge, although requiring additional processing is often done to increase the strength of
the welded joint or where thicker parts are to be welded. Although most closely
associated with a butt joint, groove welds are used on all joint types except lap.
FIGURE 1.3 Some typical groove welds: la) square groove weld, one side; (b) single
bevel groove weld; (c) single V-groove weld; (d) single U-groove weld; (e) single Igroove weld; (f) double V-groove weld for thicker sections. Dashed lines show the
original part edges.
Plug welds and slot welds are used for attaching flat plates, as shown in Figure 1.4, using
one or more holes or slots in the top part and then filling with filler metal to fuse the two
parts together.
FIGURE 1.4 (a) Plug weld, and (b) slot weld.
Spot welds and seam welds, used for lap joints, are diagrammed in Figure 1.5. A spot
weld is a small fused section between the surfaces of two sheets or plates. Multiple spot
welds are typically required to join the parts. It is most closely associated with resistance
welding. A seam weld is similar to a spot weld except it consists of a more or less
continuously fused section between the two sheets or plates.
FIGURE 1.5 (a) Spot weld, and (b) seam weld
Flange welds and surfacing welds are shown in Figure 1.6. A flange weld is made on the
edges of two (or more) parts, usually sheet metal or thin plate, at least one of the parts
being flanged as in Figure 1.6(a). A surfacing weld is not used to join parts, but rather to
deposit filler metal onto the surface of a base part in one or more weld beads. The weld
beads can be made in a series of overlapping parallel passes, thereby covering large areas
of the base part. The purpose is to increase the thickness of the plate or to provide a
protective coating on the surface.
FIGURE 1.6 (a) Flange weld, and (b) surfacing weld.
Advantages of Welding:
Welding has the following advantages
1. it produces a permanent joint
2. The overall cost of welding equipment is generally low.
3. many portable welding instruments are available
4. a large number of metals can be welded
5. a good weld is as strong as the base metal
6. Welding can be mechanized for production.
Disadvantages of welding
1. Welding creates residual stresses and the distortion in wrokpieces.
2. Edge preparation is generally required before welding.
3. A skilled welder is essential for performing a good welding operation.
4. Since welding produces internal stresses, the work piece often requires annealing
or stress-relieving.
5. Welding produces structural, physical and chemical changes.
6. Jigs and fixtures are needed to hold parts in position.
7. Welding gives off harmful radiations like light, fumes and spatters.
Pressure welding, which involves heating the ends of metal pieces to be joined to a high
temperature, but lower than their melting point and then keeping the metal pieces joined
together under pressure for sometime. This results in the pieces welding together to
produce a strong joint. There are many sub classifications of welding under each head.
Sub classification is done according to the source of heat required for fusion or pressure
welding. We shall deal with but three of them (a) Gas welding (b) Electric arc welding,
and (c) Electric resistance welding.
Cold Welding: Cold welding (CW) is a solid-slate welding process accomplished by
applying high pressure between clean contacting surfaces at room temperature. The
faying surfaces must be exceptionally clean for CW to work, and cleaning is usually done
by degreasing and wire brushing immediately before joining. Also, at least one of the
metals to be welded, and preferably both, must be very ductile and free of work
hardening. Metals such as soft aluminum and copper can be readily cold welded. The
applied compression forces in the process result in cold working of the metal parts,
reducing thickness by as much as 50%; but they also cause localized plastic deformation
at the contacting surfaces, resulting in coalescence. For small parts, the forces may be
applied by simple hand operated tools. For heavier work, powered presses are required to
exert the necessary force. No heat is applied from external sources in CW, but the
deformation process raises the temperature of the work somewhat. Applications of CW
include making electrical connections.
1.6.Welding Operation
Setting of the job: Parts to be welded are cleaned and the joint prepared. Joint
preparation depends upon the thickness of work pieces. Thin sheets can be joined by an
edge or flange-joint. Sometimes, a lap or fillet joint can be used. A sheet of higher
thickness but not exceeding 4.5 mm may be welded with a butt joint without any joint
preparation. Different kind of joints commonly used in welding are illustrated in Fig. 1.7.
Fig. 1.7 Different types of joints
For sound welding of plates thicker than 4.5 mm, detailed joint preparation is needed.
The edges of the two plates to be welded are bevelled resulting in formation of V-shaped
groove between them. The edges of the two plates are not allowed to touch each other,
instead they are separated by a gap of about 2–3 mm. If plates are thicker still, instead of
a single V-joint, a double V-joint is resorted to. A single V and a double V-joint is shown
in Fig. 1.8
Fig. 1.8 V groove joints
Gas welding techniques are classified as:
1. Left ward welding or forehand welding technique, and
2. Right ward or backhand welding technique.
The position of welding torch, filler rod and direction of welding for these techniques is
shown in Fig. 1.9.
Fig. 1.9 Welding techniques
It will be noticed that in the left ward welding technique, the flame from the torch
preheats the material yet to be welded, whereas in the right ward welding, the flame
postheats the weld-bead. This has certain metallurgical significance.
1.7.Welding Positions
These are four welding positions from the point of view of the welder. These affect
execution of sound welding.
These positions are:
1. Down hand welding position: This is the most comfortable position for welder to
work in and he is able to produce welds of a good quality.
2. Horizontal welding position (on a vertical surface).
3. Vertical welding position (on a vertical surface).
4. Overhead welding position (say on the ceiling of a room): This is the most difficult
welding position. Not only the operator has to crane his neck upwards and raise his arm
to maintain arc, it is also difficult as molten metal tends to fall down due to gravity.
For important jobs, manipulators are used, which are capable of turning over the jobs and
as much welding is done in down hand welding position as possible.
1.8.Forge Welding
Forge welding is the oldest techniques of welding which is still used, though to a limited
extent due to certain difficulties associated with this process. Generally forge welding is
carried out with the blacksmith’s sire using coal, charcoal or oil as fuels. In forge welding
care should be taken to ensure that heating of the component is optimal, as excess heat
will burn the metal and produce a brittle weld, while lesser heat will result in inadequate
welding. The preparation of joint in forge welding is shown in fig below.
Figure 1.10 Forge welding joint preparation
The process is carried out by heating the components to a plastic state, upsetting the
edges (preparation of edges) and completing the weld by hammering.
Forge welding is classified as butt, lap, L or T according to the shape of the component as
shown in the fig below.
Forge welding can be classified as
1. fire welding
2. water-gas welding
In fire welding, the pieces to be joined are heated in the blacksmith’s forge. The work
pieces are heated and withdrawn from fire at appropriate time and joined by repeated
hammer blows.
Water gas welding is used in the manufacture of pipes, containers, eetc.
tc. edges of the plate
to be converted in to pipes are heated until the appropriate temperature is attained.
The welding is completed by hammer blows or by means of pressure rollers. Borax in
combination with ammoniac is the most commonly used flux in for
forge
ge welding of steel.
Forge welding is used in rail
rail-road
road shops and repair shops of general character. It is also
used for making pipes from plates by rolling. Its use is restricted to the welding of
wrought iron and mild steel.
A correctly made forge welded joint is as strong as oxy-acetylene
acetylene joint or arc-welding
arc
joint. However, forge welding process requires considerable skill and is slow as
compared to other processes.
1.9.Resistance Welding
Resistance welding (RW) is a group of fusion
fusion-welding
welding processes that uses a combination
of heat and pressure to accomplish coalescence, the heat being generated by electrical
resistance to current Bow at the junction to be welded. The principal components in
resistance weldingg are shown in Figure 1.11 for a resistance spot-welding
welding operation, the
most widelyy used process in the group. Th
Thee components include worckparts to be welded
(usually sheet metal parts), two opposing electrodes, a means of applying pressure to
squeeze the parts between the electrodes, and an AC power supply from which a
controlled current can be applied. The operation results in a fused zone between the two
parts, called a weld nugget in spot welding. By comparison to arc welding, resistance
welding uses no shielding gases, flux, or filler metal; and the electrodes that conduct
electrical power to the process are non consumable. RW is classified as fusion welding
because the applied heat almost always causes melting of the faying surfaces. However,
there are exceptions. Some welding operations based on resistance heating use
temperatures below the melting points of the base metals, so fusion does not occur.
FIGURE 1.11 Resistance welding, showing the components in spot welding, the
predominant process in the RW group.
Resistance Spot Welding: Resistance spot welding is by far the predominant process in
this group. It is widely used in mass production of automobiles, appliances, metal
furniture, and other products made of sheet metal. If one considers that a typical car body
has approximately 10,000 individual spot welds, and that the annual production of
automobiles throughout the world is measuied in tens of millions of units, the economic
importance of resistance spot welding can he appreciated. Resistance spot welding
(RSW) is an RW process in which fusion of the faying surfaces of a lap joint is achieved
at one location by opposing electrodes. The process is used to join sheet-metal parts of
thickness 3 mm (0.125 in) or less, using a series of spot welds, in situations where an
airtight assembly is not required. The size and shape of the weld spot is determined by
the electrode tip, the most common electrode shape being round, but hexagonal, square,
and other shapes are also used. The resulting weld nugget is typically 5-10 mm (0.2-0.4
in) in diameter, with a heat-affected zone extending slightly beyond the nugget into the
base metals. If the weld is made properly, its strength will be comparable to that of the
surrounding metal. The steps in a spot welding cycle are depicted in Figure 1.12.
Materials used for RSW electrodes consist of two main groups: (1) copper-based alloys
and (2) refractory metal compositions such as copper and tungsten combinations. The
second group is noted for superior wear resistance. As in most manufacturing processes,
the tooling in spot welding gradually wears out as it is used. Whenever practical, the
electrodes are designed with internal passageways for water cooling. Because of its
widespread industrial use, various machines and methods are available to perform spotwelding operations. The equipment includes rocker-arm and press type spot-welding
machines, and portable spot-welding guns. Rocker-am spot welders, shown in Figure
1.13, have a stationary lower electrode and a movable upper electrode that can he raised
and lowered for loading and unloading the work. The upper electrode is mounted on a
rocker arm (hence the name) whose movement is controlled by a foot pedal operated by
the worker. Modern machines can be programmed to control force and current during the
weld cycle. Press-type spot welders are intended for larger work. The upper electrode
has a straight-line motion provided by a vertical press that is pneumatically or
hydraulically powered. The press action permits larger forces to be applied, and the
controls usually permit programming of complex weld cycles.
FIGURE 1.12 (a) Steps in a spot-welding cycle, and (b) plot of squeezing force and
current during cycle. The sequence is: (1) parts inserted between open electrodes, (2)
electrodes close and force is applied, (3) weld time--current is switched on, (4) current is
turned off but force is maintained or increased (a reduced current is sometimes applied
near the end of this step for stress relief in the weld region), and (5) electrodes are
opened, and the welded assembly is removed.
The previous two machine types are both stationary spot welders, in which the work is
brought to the machine. For large, heavy work it is difficult to move (and orient) the work
to stationary machines. For these cases, portable spot-welding guns are available in
various sizes and configurations. These devices consist of two opposing electrodes
contained in a pincer mechanism. Each unit is light weight so that it can be held and
manipulated by a human worker or an industrial robot. The gun is connected to its own
power and control source by means of flexible electrical cables and air hoses. Water
cooling for the electrodes, if needed, can also be provided through a water hose. Portable
spot-welding guns are widely used in automobile final assembly plants to spot weld car
bodies. Some of these guns are operated by people, but industrial robots have become the
preferred technology.
FIGURE 1.13 Rocker-arm spot-welding machine.
Resistance Seam Welding In resistance seam welding (RSEW), the stick-shaped
electrodes in spot welding are replaced by rotating wheels, as shown in Figure 1.14, and a
series of overlapping spot welds are made along the lap joint. The process is capable of
producing air-tight joints, and its industrial applications include the production of
gasoline tanks, automobile mufflers, and vaiious other fabricated sheet metal containers.
Technically, RSEW is the same as spot welding, except that the wheel electrodes
introduce certain complexities. Since the operation is usually carried out continuously,
rather than discretely, the seams should be along a straight or uniformly curved line.
Sharp corners and similar discontinuities are difficult lo deal with. Also, warping of the
parts becomes more of a factor in resistance seam welding, and well-designed fixtures are
required to hold the work in position and minimize distortion.
FIGURE 1.14 Resistance seam welding (KSEW).
The spacing between the weld nuggets in resistance seam welding depends on the motion
of the electrode wheels relative to the application of the weld current. In the usual method
of operation, called continuous motion welding, the wheel is rotated continuously at a
constant velocity, and current is turned on at timing intervals consistent with the desired
spacing between spot welds along the seam. Frequency of the current discharges is
normally set so that overlapping weld spots are produced. But if the frequency is reduced
sufficiently, then there will be spaces between the weld spots, and this method is termed
roll spot welding. In another variation, the welding current remains on at a constant level
(rather than being pulsed) so that a truly continuous welding seam is produced. These
variations arc depicted in Figure 1.15.
FIGURE 1.15 Different types of seams produced by electrode wheels: (a) conventional
resistance scam welding, in which overlapping spots are produced; (b) roll spot welding;
and (c) continuous resistance seam.
An alternative to continuous motion welding is intermittent motion welding, in which
the electrode wheel is periodically stopped to make the spot weld. The amount of wheel
rotation between stops determines the distance between weld spots along the seam,
yielding patterns similar to (a) and (b) in Figure 1.15. Seam-welding machines are similar
to press-type spot welders except that electrode wheels are used rather than the usual
stick-shaped electrodes. Cooling of the work and wheels is often necessary in RSEW, and
this is accomplished by directing water at the top and underside of the work part surfaces
near the electrode wheels.
Resistance Projection Welding Resistance projection welding (RPW) is an RW
process in which coalescence occurs at one or more relatively small contact points on the
parts. These contact points are determined by the design of the parts to be joined, and
may consist of projections, embossments, or localized intersections of the parts. A typical
case in which two sheet-metal parts are welded together is described in Figure 1.16. The
part on top has been fabricated with two embossed points to contact the other part at the
start of the process. It might be argued that the embossing operation increases the cost of
the part, but this increase may be more than offset by savings in welding cost.
FIGURE 1.16 Resistance projection welding (RPW): (1) at start of operation, contact
between parts is at projections; and (2) when current is applied, weld nuggets similar to
those in spot welding are formed at the projections.
There are variations of resistance projection welding, two of which are shown in Figure
1.17. In one variation, fasteners with machined or formed projections can be permanently
joined to sheet or plate by RPW, facilitating subsequent assembly operations. Another
variation, called cross-wire welding, is used to fabricate welded wire products such as
wire fence, shopping carts, and stove grills. In this process, the contacting surfaces of the
round wires serve as the projections to localize the resistance heat for welding.
FIGURE 1.17 Two variations of resistance projection welding: (a) welding of a
machined or formed fastener onto a sheet-metal part; and (b) cross-wire welding.
1.10.
Butt Welding Process
Welding two pieces of metal together, end to end, is called butt welding. In butt welding
the ends are cleaned and made square so that the two pieces touch each other over the
entire cross-section. One piece is held in stationary clamps (Refer to Fig. 1.18) and the
other piece in movable clamp.
The movable clamps bring the two pieces to be welded together end to end. Then the
current is switched on heating the ends quickly. Then the movable be clamps close in
with pressure and hold the two pieces together under pressure until the butt weld is made.
Obviously, the material around the joint upsets and has to be cut and thrown away.
Fig. 1.18 Butt welding (ERW)
Percussion welding (PEW) is also similar to flash welding, except that the duration of
the weld cycle is extremely short, typically lasting only 1-10 ms. Fast heating is
accomplished by rapid discharge of electrical energy between the two surfaces to he
joined, followed immediately by percussion of one part against the other to form the
weld. The heating is very localized, making this process attractive for electronic
applications in which the dimensions are very small and nearby components may be
sensitive to heat.
1.11.
Arc Welding
Arc welding (AW) is a fusion-welding process in which coalescence of the metals is
achieved by the heat from an electric arc between an electrode and the work. The same
basic process is also used in arc cutting. A generic AW process is shown in Figure 1.19.
An electric arc is a discharge of electric current across a gap in a circuit. It is sustained by
the presence of a thermally ionized column of gas (called plasma) through which current
flows. To initiate the arc in an AW process, the electrode is brought into contact with the
work and then quickly separated from it by a short distance. The electric energy from the
arc thus formed produces temperatures 5500°C (10,000"F) or higher, sufficiently hot to
melt any metal. A pool of molten metal, consisting of base metal(s) and filler metal (if
one is used) is formed near the tip of the electrode. In most arc-welding processes, filler
metal is added during the operation lo increase the volume and strength of the weld joint.
As the electrode is moved along the joint, the molten weld pool solidifies in its wake.
FIGURE 1.19. The basic configuration and electrical circuit of an arc-welding process
Movement of the electrode relative to the work is accomplished by either a human welder
(manual welding) or by mechanical means (i.e., machine welding, automatic welding, or
robotic welding). One of the troublesome aspects of manual arc welding is that the
quality of the weld joint depends on the skill and work ethic of the human welder.
Productivity is also an issue. It is often measured as are rime (also called arc-on lime)-the
proportion of the hours worked that arc welding is being accomplished:
Arc time = (time arc is on)/(hours worked)
This definition can be applied to an individual welder or to a mechanized workstation.
For manual welding, arc time is usually around 20°/0. Frequent rest periods are needed
by the welder to overcome fatigue in manual arc welding, which is demanding in handeye coordination under stressful conditions. Arc time increases to about 50% (more or
less, depending on the operation) for machine, automatic, and robotic welding.
Polarity in arc welding:
There is no polarity in A.C. arc welding due to the reversal of the current, or the heat
generated at each pole is the same and therefore, changing over the connections to the
electrode and job does not have may effect on the performance of the welding. Polarity is
a significant factor in all the D.C. welding processes. This polarity is of two types:
1. Straight polarity: when the electrode is connected to the negative terminal and the
work piece to positive.
2. Reverse polarity: in this case the workpiece is connected to a negative terminal
and the electrode to a positive.
These two polarities are also known as electrode negative and electrode positive
respectively. The heat distribution generated by the flow of the current split in to two
parts, i.e, 2/3 at the positive terminator or pole and 1/3 at the negative pole. So the
selection of correct polarity plays a significant role in order to obtain a successful weld. It
is only due to this factor that the majority, in fact almost all, metals requires more heat to
reach the fusion state than the electrode e.g. copper and its alloys.
Comparison between AC and DC arc welding
Sl. No
Aspects
A.C. Welding
D.C. Welding
1
Power consumption
Low
High
2
Arc stability
Unstable
Stable
3
Cost
Less
More
4
Weight
Light
Heavy
5
Efficiency
High
Low
6
Operation
Noiseless
Noisy
7
8
9
Non-ferrous metals cannot be
Suitability
joined
Electrode used
Welding
of
sections
Only coated
thin
Not preferred
Suitable
for
both
ferrous
and
non-
ferrous metals
Bare electrode are
also used
Preferred
Work can act as cathode while Electrode is always
10
miscellaneous
electrode acts as anode and vice negative
versa
and
the
work is positive.
Electrodes: Electrodes used in AW processes are classified as consumable or non
consumable. Consumable electrodes provide the source of the liller metal in arc welding.
These electrodes are available in two principal forms: rods (also called sticks) and wire.
Welding rods are typically 225450 mm (9-18 in) long and 9.5 mm (318 in) or less in
diameter. The problem with consumable welding rods, at least in production welding
operations, is that they must be changed periodically, reducing arc time of the welder.
Consumable weld wire has the advantage that it can be continuously fed into the weld
pool from spools containing long lengths of wire, thus avoiding the frequent interruptions
that occur when using welding sticks. In both rod and wire forms, the electrode is
consumed by the arc during the welding process and added to the weld joint as liller
metal.
Non consumable electrodes are made of tungsten (or carbon, rarely), which resists
melting by the arc. Despite its name, a non consumable electrode is gradually depleted
during the welding process (vaporization is the principal mechanism), analogous to the
gradual wearing of a cutting tool in a machining operation. For AW processes that utilize
non consumable electrodes, any filler metal used in the operation must be supplied by
means of a separate wire that is fed into the weld pool.
Carbon arc welding (CAW) is an arc-welding process in which a non consumable
carbon (graphite) electrode is used. It has historical importance because it was the first
arc-welding process to be developed, but its commercial importance today is practically
nil. The carbon arc process is used as a beat source for brazing and for repairing iron
castings. It can also be used in some applications for depositing wear-resistant materials
on surfaces. Graphite electrodes for welding have been largely superseded by tungsten (in
GTAW and PAW).
Shielded Metal Arc Welding Shielded metal arc welding (SMAW) is an AW process
that uses a consumable electrode consisting of a filler metal rod coated with chemicals
that provide flux and shielding. The process is illustrated in Figures 1.20 and 1.21. The
welding stick (SMAW is sometimes called stick welding) is typically 225-450 mm (9-18
in) long and 2.5-9.5 mm (3132-318 in) in diameter. The filler metal used in the rod must
be compatible with the metal to be welded, the composition usually being very close to
that of the base metal. The coating consists of powdered cellulose (i.e., cotton and wood
powders) mixed with oxides, carbonates, and other ingredients, held together by a silicate
binder. Metal powders are also sometimes included in the coating to increase the amount
of filler metal and to add alloying elements. The heat of the welding process melts the
coating to provide a protective atmosphere and slag for the welding operation. It also
helps to stabilize the arc and regulate the rate at which the electrode melts. During
operation the bare metal end of the weld link stick (opposite the welding tip) is clamped
in an electrode holder that is connected to the power source. The holder has an insulated
handle so that it can be held and manipulated by a human welder. Currents typically used
in SMAW range between 30 and 300 A at voltages from 15 to 45 V. Selection of the
proper power parameters depends on the metals being welded, electrode type and length',
and depth of weld penetration required. Power supply, connecting cables, and electrode
holder can be bought for a few thousand dollars.
Shielded metal arc welding is usually performed manually. Common applications include
construction, pipelines, machinery structures, shipbuilding, fabrication job shops, and
repair work. It is preferred over oxy fuel welding for thicker sections-above 5 mm (3116
in)-because of its higher power density. The equipment is portable and low cost, making
SMAW highly versatile and probably the most widely used of the AW processes. Base
metals include steels, stainless steels, cast irons, and certain nonferrous alloys. It is not
used or seldom used for aluminum and its alloys, copper alloys, and titanium.
A disadvantage of shielded metal arc welding as a production operation is the use of the
consumable electrode stick. As the sticks are used up, they must periodically be changed.
This reduces the arc time with this welding process. Another limitation is the current
level that can be used. Because the electrode length varies during the operation and this
length affects the resistance heating of the electrode, current levels must be maintained
within a safe range or the coating will overheat and melt prematurely when starting a new
welding stick. Some of the other AW processes overcome the limitations of welding stick
length in SMAW by using a continuously fed wire electrode.
FIGURE 1.20 Shielded metal arc welding (SMAW).
Gas Metal Arc Welding: Gas metal arc welding (GMAW) is an AW process in which
the electrode is a consumable bare metal wire, and shielding is accomplished by flooding
the arc with a gas. The bare wire is fed continuously and automatically from a spool
through the welding gun, as illustrated in Figure 1.21. Wire diameters ranging from 0.8 to
6.5 mm (1132-114 in) are used in GMAW, the size depending on the thickness of the
parts being joined and the desired deposition rate. Gases used for shielding include inert
gases such as argon and helium, and active gases such as carbon dioxide. Selection of
gases (and mixtures of gases) depends on the metal being welded, as well as other
factors. Inert gases are used for welding aluminum alloys and stainless steels, while CO2
is commonly used for welding low and medium carbon steels. The combination of bare
electrode wire and shielding gases eliminates the slag covering on the weld bead and thus
precludes the need for manual grinding and cleaning of the slag. The GMAW process is
therefore ideal for making multiple welding passes on the same joint.
FIGURE 1.21 Gas metal arc welding (GMAW).
The various metals on which GMAW is used and thc variations of the process itself have
given rise to a variety of names for gas metal arc welding. When the process was first
introduced in the late 1940s, it was applied to the welding of aluminum using inert gas
(argon) for arc shielding. The name applied to this process was MIG welding (for metal
inert gas welding). When the same welding process was applied to steel, it was found that
inert gases were expensive and C02 was used as a substitute. Hence the term CO2
welding was applied. Refinements in GMAW for steel welding have led to the use of gas
mixtures, including CO2 and argon, and even oxygen and argon. GMAW is widely used
in fabrication operations in factories for welding a variety of ferrous and nonferrous
metals. Because it uses continuous weld wire rather than welding sticks, it has a
significant advantage over SMAW in terms of arc time when performed manually. For
the same reason, it also lends itself to automation of arc welding. The electrode stubs
remaining after stick welding also wastes tiller metal, so the utilization of electrode
material is higher with GMAW Other features of GMAW include elimination of slag
removal (since no flux is used), higher deposition rates than SMAW, and good versatility.
Flux-Cored Arc Welding This arc-welding process was developed in the early 1950s as
an adaptation of shielded metal arc welding to overcome the limitations imposed by the
use of stick electrodes. Flax-cored arc welding (FCAW) is an arc-welding process in
which the electrode is i~ continuous consumable tubing that contains flux and other
ingredients in its core. Other ingredients may include deoxidizes and alloying elements.
The tubular flux-cored "wire" is flexible and can therefore be supplied in the form of
coils to be continuously fed through the arc-welding gun. There are two versions of
FCAW (1) self-shielded and (2) gas shielded. In the first version of FCAW to be
developed, arc shielding was provided by a flux core, thus leading to the name selfshielded flux-cored arc welding . The core in this form of FCAW includes not only
fluxes but also ingredients that generate shielding gases for protecting the sic. The second
version of FCAW, developed primarily for welding steels, obtains arc shielding from
externally supplied gases, similar to gas metal arc welding. This version is called gasshielded flux-cored arc welding. Because it utilizes an electrode containing its own flux
together with separate shielding gases, it might be considered a hybrid of SMAW and
GMAW. Shielding gases typically employed are carbon dioxide for mild steels or
mixtures of argon and carbon dioxide for stainless steels. Figure 1.22 illustrates the
FCAW process, with the gas (optional) distinguishing between the two types.
FCAW has advantages similar to GMAW, due to continuous feeding of the electrode. It
is used primarily for welding steels and stainless steels over a wide stock thickness range.
It is noted for its capability to produce very-high-quality weld joints that are smooth and
uniform
FIGURE 1.22 Flux-cored arc welding. The presence or absence of externally supplied
shielding gas distinguishes the two types: (1) self-shielded, in which the core provides the
ingredients for shielding; and (2) gas shielded, in which external shielding gases are
supplied.
Electro gas Welding: Electro gas welding (EGW) is an AW process that uses a
continuous consumable electrode (either flux-cored wire or bare wire with externally
supplied shielding gases) and molding shoes to contain the molten metal. The process is
primarily applied to vertical butt welding, as pictured in Figure 1.23. When the flux-cored
electrode wire is employed, no external gases are supplied, and the process can be
considered a special application of self-shielded FCAW. When a bare electrode wire is
used with shielding gases from an external source, it is considered a special case of
GMAW. The molding shoes are water cooled to prevent their being added to the weld
pool. Together with the edges of the parts being welded, the shoes form a container,
almost like a mold cavity, into which the molten metal from the electrode and base parts
is gradually added. The process is performed automatically, with a moving weld head to
travel vertically upward to fill the cavity in a single pass. Principal applications of electro
gas welding are steels (low-and medium-carbon, low-alloy, and certain stainless steels) in
the construction of large storage tanks and in shipbuilding. Stock thick nesses from 12 to
75 mm (0.5-3.0 in) are within the capacity of EGW. In addition to butt welding, it can
also be used for fillet and groove welds, always in a vertical orientation. Specially
designed molding shoes must sometimes be fabricated for the joint shapes involved.
FIGURE 1.23 Electro gas welding using flux-cored electrode wire: (a) front view with
molding shoe removed for clarity, and (b) side view showing molding shoes on both
sides.
Submerged Arc welding: This process, developed during the 1930s, was one of the first
AW processes to be automated. Submerged arc welding (SAW) is an arc-welding
process that uses a continuous, consumable bare wire electrode, and arc shielding is
provided by a cover of granular flux. The electrode wire is fed automatically from a coil
into the arc. The flux is introduced into the joint slightly ahead of the weld arc by gravity
from a hopper, as shown in Figure 1.24. The blanket of granular flux completely
submerges the welding operation, preventing sparks, spatter, and radiation that are so
hazardous in other AW processes. Thus, the welding operator in SAW need not wear the
somewhat cumbersome face shield required in the other operations (safety glasses and
protective gloves, of course, are required). The portion of the flux closest to the arc is
melted, mixing with the molten weld metal to remove impurities and then solidifying on
too of the weld joint to form a glass-like slag. The slag and un fused flux granules on top
provide good protection from the atmosphere and good thermal insulation for the weld
area, resulting in relatively slow cooling and a high-quality weld joint, noted for
toughness and ductility. As depicted in our sketch, the unfused flux remaining after
welding can be recovered and reused. The solid slag covering the weld must be chipped
away, usually by manual means.
FIGURE 1.24 Submerged arc welding.
Gas Tungsten Arc Welding Gas tungsten arc welding (GTAW) is an AW process that
uses a non consumable tungsten electrode and an inert gas for arc shielding. The term
TIG welding (tungsten inert gas welding) is often applied lo this process (in Europe, WIG
welding is the term-the chemical symbol for tungsten is W, for Wolfram). The GTAW
process can be implemented with or without a filler metal. Figure 1.25 illustrates the
latter case. When a filler metal is used, it is added to the weld pool from a separate rod or
wire, being melted by the heat of the, arc rather than transferred across the arc as in the
consumable electrode AW processes. Tungsten is a good electrode material due to its
high melting point of 3410°C (6170°F). Typical shielding gases include argon, helium, or
a mixture of these gas elements. GTAW is applicable to nearly all metals in a wide range
of stock thicknesses. It can also be used for joining various combinations of dissimilar
metals. Its most common applications are for aluminum and stainless steel. Cast irons,
wrought irons, lead, and of course tungsten are difficult to weld by GTAW In steel
welding applications, GTAW is generally slower and more costly than the consumable
electrode AW processes, except when thin sections are involved and very-high-quality
welds are required. When thin sheets are TIG welded to close tolerances, filler metal is
usually not added. The process can be performed manually or by machine and automated
methods for all joint types. Advantages of GTAW in the applications to which it is suited
include high-quality welds, no weld spatter because no filler metal is transferred across
the arc, and little or no post weld cleaning because no flux is used.
FIGURE 1.25 Gas tungsten arc welding
Plasma Arc Welding Plasma arc welding (PAW) is a special form of gas tungsten arc
welding in which a constricted plasma arc is directed at the weld area. In PAW, a
tungsten electrode is contained in a specially designed nozzle that focuses a high-velocity
stream of inert gas (e.g., argon or argon-hydrogen mixtures) into the region of the arc to
form a high-velocity, intensely hot plasma arc stream, as in Figure 1.26. Argon, argonhydrogen, and helium are also used as the arc-shielding gases.
Temperatures in plasma arc welding reach 28,000" (50,000°F) or greater, hot enough to
melt any known metal. The reason why temperatures are so high in PAW (significantly
higher than those in GTAW) derives from the constriction of the arc. Although the
typical power levels used in PAW are below those used in GTAW, the power is highly
concentrated to produce a plasma jet of small diameter and very high power density.
Plasma arc welding was introduced around 1960 but was slow to catch on. In recent years
its use is increasing as a substitute for GTAW in applications such as automobile
subassemblies, metal cabinets, door and window frames, and home appliances. Owing to
the special features of PAW, its advantages in these applications include good arc
stability, better penetration control than most other AW processes, high travel speeds, and
excellent weld quality. The process can be used to weld almost any metal, including
tungsten. Difficult-to-weld metals with PAW include bronze, cast irons, lead, and
magnesium. Other limitations include high equipment cost and larger torch size than
other AW operations, which tends to restrict access in some joint configurations.
FIGURE 1.26 Plasma arc welding (PAW)
Stud welding (SW) is a specialized AW process for joining studs or similar components
to base parts. A typical SW operation is illustrated in Figure 1.27, in which shielding is
obtained by the use of a ceramic ferrule. To begin with, the study is chucked in a special
weld gun that automatically controls the timing and power parameters of the steps shown
in the sequence. The worker must only position the gun at the proper location against the
base workpart to which the study will be attached and pull the trigger. SW applications
include threaded fasteners for attaching handles to cookware, heat radiation fins on
machinery, and similar assembly situations. In high-production operations, stud welding
usually has advantages over rivets, manually arc-welded attachments, and drilled and
tapped holes.
Laser-Beam Welding
Laser-beam welding (LBW) is a fusion-welding process in which coalescence is achieved
by the energy of a highly concentrated, coherent light beam focused on the joint to be
welded. The term laser is an acronym for light amplification by stimulated emission of
radiation. This same technology is used for laser-beam machining. LBW is normally
performed with shielding gases (e.g., helium, argon, nitrogen, and carbon dioxide) to
prevent oxidation. Filler metal is not usually added. LBW produces welds of high quality,
deep penetration, and narrow heat-affected zone. These features are similar to those
achieved in electron-beam welding, and the two processes are often compared. There are
several advantages of LBW over EBW: no vacuum chamber is required, no X-rays are
emitted, and laser beams can be focused and directed by optical lenses and mirrors. On
the other hand, LBW does not possess the capability for the deep welds and high depthto-width ratios of EBW. Maximum depth in laser welding is about 19 mm (0.75 in),
whereas EBW can be used for weld depths of 50 mm (2 in) or more; and the depth-towidth ratios in LBW are typically limited to around 5:l. Because of the highly
concentrated energy in the small area of the laser beam, the process is often used to join
small parts.
Atomic hydrogen welding (AHW) is an arc welding process that uses an arc between
two metal tungsten electrodes in a shielding atmosphere of hydrogen. The process was
invented by Irving Langmuir in the course of his studies of atomic hydrogen. The electric
arc efficiently breaks up the hydrogen molecules, which later recombine with tremendous
release of heat, reaching temperatures from 3400 to 4000 °C. Without the arc, an
oxyhydrogen torch can only reach 2800 °C. This is the third hottest flame after
cyanogen at 4525 °C and dicyanoacetylene at 4987 °C. An acetylene torch merely
reaches 3300 °C. This device may be called an atomic hydrogen torch, nascent
hydrogen torch or Langmuir torch. The process was also known as arc-atom welding.
The heat produced by this torch is sufficient to melt and weld tungsten (3422 °C), the
most refractory metal. The presence of hydrogen also acts as a gas shield and protects
metals from contamination by carbon, nitrogen, or oxygen, which can severely damage
the properties of many metals. It eliminates the need of flux for this purpose.
The arc is maintained independently of the workpiece or parts being welded. The
hydrogen gas is normally diatomic (H2), but where the temperatures are over 600 °C
(1100 °F) near the arc, the hydrogen breaks down into its atomic form, simultaneously
absorbing a large amount of heat from the arc. When the hydrogen strikes a relatively
cold surface (i.e., the weld zone), it recombines into its diatomic form and rapidly
releases the stored heat. The energy in AHW can be varied easily by changing the
distance between the arc stream and the workpiece surface. This process is being replaced
by shielded metal-arc welding, mainly because of the availability of inexpensive inert
gases.
In atomic hydrogen welding, filler metal may or may not be used. In this process, the arc
is maintained entirely independent of the work or parts being welded. The work is a part
of the electrical circuit only to the extent that a portion of the arc comes in contact with
the work, at which time a voltage exists between the work and each electrode.
1.12.
Electroslag Welding
Electroslag welding (ESW) uses the same.basic equipment as some of the arc-welding
processes, and it utilizes an arc to initiate the welding operation. However, it is not an
AW process because an arc is not used during welding. Electroslag welding (ESW) is a
fusion-welding process in which coalescence is achieved by hot, electrically conductive
molten slag acting on the base parts and filler metal. As shown in Figure 1.28, the general
configuration of ESW is similar to eleclrogas welding. It is performed in a vertical
orientation (shown here for butt welding), using water-cooled molding shoes to contain
the molten slag and weld metal. At the start of the process, granulated conductive flux is
put into the cavity. The consumable electrode tip is positioned near the bottom of the
cavity, and an arc is generated for a short while to start melting the flux. Once a pool of
slag has been created, the arc is extinguished and the current passes from the electrode to
the base metal through the conductive slag, so that its electrical resistance generates heat
to maintain the welding process. Since the density of the slag is less than that of the
molten metal, it remains on top to protect the weld pool. Solidification occurs from the
bottom, while additional molten metal is supplied from above by the electrode and the
edges of the base parts. The process gradually continues until it reaches the top of the
joint.
FIGURE 1.28 Electroslag welding (ESW): (a) front view with molding shoe removed for
clarity; (b) side view showing schematic of molding shoe. Setup is similar to electragas
welding except that resistance heating of molten slag is used to melt the base and filler
metals.
Thermit Welding
Thermit is a trademark name for thermite, a mixture of aluminum powder and iron oxide
that produces an exothermic reaction when ignited. It is used in incendiary bombs and for
welding.
As a welding process, the use of Thermit dates from around 1900. Thermit welding (TW)
is a fusion-welding process in which the heat for coalescence is produced by superheated
molten metal from the chemical reaction of Thermit. Filler metal is obtained from the
liquid metal; and although the process is used for joining, it has more in common with
casting than it does with welding. Finely mixed powders of aluminum and iron oxide (in
a 1:3 mixture), when ignited at a temperature of around 1300°C (2300oF), produce the
following chemical reaction:
8A1+ 3Fe304 →9Fe + 4AI2O1 +heat
The temperature from the reaction is around 2500°C (4500°F), resulting in superheated
molten iron plus aluminum oxide that floats to the top as a slag and protects the iron from
the atmosphere. In thermit welding, the superheated iron (or steel if the mixture of
powders is formulated accordingly) is contained in a crucible located above the joint to
be welded, as indicated by our diagram of the TW process in Figure 1.29. After the
reaction is complete (about 30 s, irrespective of the amount of thermit involved), the
crucible is tapped and the liquid metal flows into a mold built specially to surround the
weld joint. Because the entering metal is so hot, it melts the edges of the base parts,
causing coalescence upon solidification. After cooling, the mold is broken away, and the
gates and risers are removed by oxyacetylene torch or other method. Thermit welding has
applications in joining of railroad rails (as pictured in our figure), and repair of cracks in
large steel castings and forgings such as ingot molds, large diameter shafts, frames for
machinery, and ship rudders. The surface of the weld in these applications is often
sufficiently smooth so that no subsequent finishing is required.
Figure 1.29 thermit welding: (1) therrnit ignited; (2) crucible tapped, superheated metal
flows into mold; (3) metal solidifies to produce weld joint.
1.13.
Solid-State Welding
In solid state-welding, coalescence of the part surfaces is achieved by (1) pressure alone,
or (2) heat and pressure. For some solid-state processes, time is also a factor. If both heat
and pressure are used, the amount of heat by itself is not sufficient to cause melting of the
work surfaces. In other words, fusion of the parts would not occur using only the heat that
is externally applied in these processes. In some cases, the combination of heat and
pressure, or the particular manner in which pressure alone is applied, generates sufficient
energy to cause localized melting of the faying surfaces. Filler metal is not added in
solid-state welding.
Basic Weld Symbols
(1) General. Weld symbols are used to indicate the following welding processes used in
metal joining operations; whether the weld is localized or "all around"; shop
or field welds; and
the contour of welds. These basic weld symbols are summarized in paragraphs (2) throug
h (5) below and are illustrated in figure 4 on the following page.
(2) Arc and Gas Weld Symbols. These symbols are used as shown in figure 3, view A.
(3) Resistance Weld Symbols. These symbols are used as shown in figure 3, view B.(4) B
razing, Forge, Thermit, Induction, and Flow Weld Symbols.
(4) Brazing, Forge, Thermit, Induction, and Flow Weld Symbols
These welds are indicated by using a process or specification reference in the tail of the
welding symbols as shown in figure 4, view A. when the use of a definite process is
required (figure 4, view B), the process may be indicated by one or more of the letter is
designations shown in Tables 1 and 2 on the following pages. When no specification,
process or other reference is used with a welding symbol, the tail may be omitted, as
shown in figure, view C.
Supplementary Symbols: In addition to basic weld symbols, a set of supple-mentary
symbols may be added to a welding symbol. Some of the most common supplementary
symbols are shown in figure 1-30.Contour symbols are used with weld symbols to show
how the face of the weld is to be formed. In addition to contour symbols, finish
symbols are used to indicate the method to use for forming the contour of theweld. When
a finish symbol is used, it shows the method of finish, not the degree of finish; for
example, a C isused to indicate finish by chipping, an M means machining, and a G
indicates grinding. Figure 1-31 shows how contour and finish symbols are applied to a
welding symbol. This figure shows that the weld is to be ground flush. Also, notice that
the symbols are placed on the same side of the reference line as the weld symbol
Elements of a welding symbol: A distinction is made between the term “weld symbol”
and “welding symbol”. The “weld symbol” is the ideograph that is used to indicate the
desired type of weld. The assembled welding symbol consists of the following eight
elements or any of these elements as are necessary;
1. reference line
2. arrow
3. basic weld symbols
4. dimensions and other data
5. supplementary symbols
6. finish symbols
7. tail
8. specification process or other reference.
The location of the elements of a welding symbol with respect to each other is shown
1.14.
Oxyfuel Gas Welding
Oxyfuelgas welding (OFW) is the term used to describe the group of FW operations that
burn various fuels mixed with oxygen to perform welding. The OFW processes employ
several types of gases, which is the primary distinction among the members of this group.
Oxyfuel gas is also commonly used in cutting torches to cut and separate metal plates and
other parts. The most important OFW process is oxyacetylene welding.
Oxyacetylene welding (OAW) is a fusion-welding process performed by a high
temperature flame from combustion of acetylene and oxygen. The flame is directed by a
welding torch. A filler metal is sometimes added, and pressure is occasionally applied in
OAW between the contacting part surfaces. A typical OAW operation is sketched in
Figure 1.32. When filler metal is used, it is typically in the form of a rod with diameters
ranging from 1.6 to 9.5 mm (1116-318 in). Composition of the filler must he similar to
that of the base metals. The filler is often coated with aJ7ux that helps to clean the
surfaces and prevent oxidation, thus creating a better weld joint.
FIGURE 1.32 A typical oxyacetylene welding operation (OAW).
Acetylene (C2H2) is the most popular fuel among the OFW group because it is capable
of higher temperatures than any of the others-up to 3480°C (6300°F). The flame in OAW
is produced by the chemical reaction of acetylene and oxygen in two stages. The first
stage is defined by the reaction
C2H2 + O2 → 2CO + H2 + heat
the products of which are both combustible, which leads to the second-stage reaction
2CO + H2 + 1.5 O2 → 2C0 2+ H 2 0+ heat
The two stages of combustion are visible in the oxyacetylene flame emitted from the
torch. When the mixture of acetylene and oxygen is in the ratio 1:1, as described in Eq.
(31.4), the resulting neutralflame is shown in Figure 1.33. The first-stage reaction is
seen as the inner cone of the flame (which is bright white), while the second-stage
reaction is exhibited by the outer envelope (which is nearly colorless but with tinges
ranging from blue to orange). The maximum temperature of the flame is reached at the
tip of the inner cone; the second-stage temperatures are somewhat below those of the
inner cone. During welding, the outer envelope spreads out and covers the work surfaces
being joined, thus shielding them from the surrounding atmosphere.
Total heat liberated during the two stages of combustion is 55 x lo6 J/m3 (1470 Btu1ft3)
of acetylene. However, because of the temperature distribution in the flame, the way in
which the flame spreads over the work surface, and losses to the air, power densities and
heat transfer factors in oxyacetylene welding are relatively low; f1 = 0.10-0.30.
FIGURE 1.33 The neutral flame from an oxyacetylene torch, indicating temperatures
achieved
The apparatus used in gas welding consists basically of an oxygen source and a fuel gas
source (usually cylinders), two pressure regulators and two flexible hoses (one of each for
each cylinder), and a torch. This sort of torch can also be used for soldering and brazing.
The cylinders are often carried in a special wheeled trolley.
There have been examples of oxy hydrogen cutting sets with small (scuba-sized) gas
cylinders worn on the user's back in a backpack harness, for rescue work and similar.
There are also examples of pressurized liquid fuel cutting torches, usually using gasoline.
These are used for their increased portability.
Regulator
The regulator is used to control pressure from the tanks to the required pressure in the
hose. The flow rate is then adjusted by the operator using needle valves on the torch.
Accurate flow control with a needle valve relies on a constant inlet pressure to it.
Most regulators have two stages: the first stage of the regulator is a fixed-pressure
regulator whose function is to release the gas from the cylinder at a constant intermediate
pressure, despite the pressure in the cylinder falling as the gas in the cylinder is used.
This is similar to the first stage of a scuba-diving regulator. The adjustable second stage
of the regulator controls the pressure reduction from the intermediate pressure to the low
outlet pressure. The regulator has two pressure gauges, one indicating cylinder pressure,
the other indicating hose pressure. The adjustment knob of the regulator is sometimes
roughly calibrated for pressure, but an accurate setting requires observation of the gauge.
Some simpler or cheaper oxygen-fuel regulators have only a single stage regulator, or
only a single gauge. A single-stage regulator will tend to reduce its outlet pressure as the
cylinder is emptied, requiring manual readjustment. For low-volume users, this is an
acceptable simplification. Welding regulators, unlike simpler LPG heating regulators,
retain their outlet (hose) pressure gauge and do not rely on the calibration of the
adjustment knob. The cheaper single-stage regulators may sometimes omit the cylinder
contents gauge, or replace the accurate dial gauge with a cheaper and less precise "rising
button" gauge.
Gas hoses
The hoses are specifically designed for welding and cutting. The hose is usually a doublehose design, meaning that there are two hoses joined together. These hoses are colourcoded for visual identification and their threaded connectors are handed to avoid
accidental mis-connection: oxygen is right-handed as normal, fuel gases use a left-handed
thread. These left-handed threads also have an identifying groove cut into their nuts.
Colour coding of hoses varies between countries. In the USA, oxygen is green, and the
fuel hose is red In the UK, the oxygen hose is blue (black hoses may still be found on old
equipment), and the acetylene fuel hose is red Where LPG fuel, such as propane, is used,
the fuel hose should be orange, indicating that it is compatible with LPG. LPG will
damage an incompatible hose, including most acetylene hoses.
Connections between flexible hoses and rigid fittings are made by a crimped hose
clip over a barbed spigot. The use of worm-drive or Jubilee clips is specifically forbidden
in the UK. The hoses should also be clipped together at intervals approximately 3 feet
apart.
Non-return valve
Between the regulator and hose, and ideally between hose and torch on both oxygen and
fuel lines, a flashback arrestor and/or non-return valve (check valve) should be installed
to prevent flame or oxygen-fuel mixture being pushed back into either cylinder and
damaging the equipment or making a cylinder explode.
European practice is to fit flashback arrestors at the regulator and check valves at the
torch. US practice is to fit both at the regulator.
The flashback arrestor (not to be confused with a check valve) prevents the shock
waves from downstream coming back up the hoses and entering the cylinder (possibly
rupturing it), as there are quantities of fuel/oxygen mixtures inside parts of the equipment
(specifically within the mixer and blowpipe/nozzle) that may explode if the equipment is
incorrectly shut down; and acetylene decomposes at excessive pressures or temperatures.
The flashback arrestor will remain switched off until someone resets it, in case the
pressure wave created a leak downstream of the arrestor.
Check valve
A check valve lets gas flow in one direction only. Not to be confused with a flashback
arrestor, a check valve is not designed to block a shock wave. The pressure wave could
occur while the ball is so far from the inlet that the pressure wave gets past before the ball
reaches its off position. A check valve is usually a chamber containing a ball that is
pressed against one end by a spring: gas flow one way pushes the ball out of the way, and
no flow or flow the other way lets the spring push the ball into the inlet, blocking it.
Torches
The torch is the part that the welder holds and manipulates to make the weld. It has a
connection and valve for the fuel gas and a connection and valve for the oxygen, a handle
for the welder to grasp, a mixing chamber (set at an angle) where the fuel gas and oxygen
mix, with a tip where the flame forms.
Welding torch
A welding torch head is used to weld metals. It can be identified by having only one or
two pipes running to the nozzle and no oxygen-blast trigger and two valve knobs at the
bottom of the handle letting the operator adjust the oxygen flow and fuel flow.
Cutting torch
A cutting torch head is used to cut materials. It is similar to a welding torch, but can be
identified by the oxygen blow out trigger or lever.
The metal is first heated by the flame until it is cherry red. Once this temperature is
attained, oxygen is supplied to the heated parts by pressing the "oxygen-blast trigger".
This oxygen reacts with the metal, forming iron oxide and producing heat. It is this heat
which continues the cutting process. The cutting torch only heats the metal to start the
process; further heat is provided by the burning metal.
The melting point of the iron oxide is around half of that of the metal; as the metal burns,
it immediately turns to liquid iron oxide and flows away from the cutting zone. However,
some of the iron oxide remains on the work piece, forming a hard "slag" which can be
removed by gentle tapping, and/or a grinder.
Rose-bud torch
A rose-bud torch is used to heat metals for bending, straightening, etc. where a large area
needs to be heated. It is called as such because the flame at the end looks like a rose-bud.
A welding torch can also be used to heat small area such as rusted nuts and bolts.
Injector torch
A typical oxy-fuel torch, called an equal-pressure torch, merely mixes the two gases. In
an injector torch, high pressure oxygen comes out of a small nozzle inside the torch head
so that it drags the fuel gas along with it, via venturi effect.
Cutting
For cutting, the set-up is a little different. A cutting torch has a 60- or 90-degree angled
head with orifices placed around a central jet. The outer jets are for preheat flames of
oxygen and acetylene. The central jet carries only oxygen for cutting. The use of a
number of preheating flames, rather than a single flame makes it possible to change the
direction of the cut as desired without changing the position of the nozzle or the angle
which the torch makes with the direction of the cut, as well as giving a better preheat
balance. Manufacturers have developed custom tips for Mapp, propane, and
polypropylene gases to optimize the flames from these alternate fuel gases.
The flame is not intended to melt the metal, but to bring it to its ignition temperature.
The torch's trigger blows extra oxygen at higher pressures down the torch's third tube out
of the central jet into the workpiece, causing the metal to burn and blowing the resulting
molten oxide through to the other side. The ideal kerf is a narrow gap with a sharp edge
on either side of the workpiece; overheating the workpiece and thus melting through it
causes a rounded edge.
Cutting is initiated by heating the edge or leading face (as in cutting shapes such as round
rod) of the steel to the ignition temperature (approximately bright cherry red heat) using
the pre-heat jets only, then using the separate cutting oxygen valve to release the oxygen
from the central jet.. The oxygen chemically combines with the iron in the ferrous
material to instantly oxidize the iron into molten iron oxide, producing the cut. Initiating
a cut in the middle of a workpiece is known as piercing.
It is worth noting several things at this point:
The oxygen flowrate is critical — too little will make a slow ragged cut; too much
will waste oxygen and produce a wide concave cut. Oxygen Lances and other
custom made torches do not have a separate pressure control for the cutting
oxygen, so the cutting oxygen pressure must be controlled using the oxygen
regulator. The oxygen cutting pressure should match the cutting tip oxygen
orifice. Consult the tip manufacturer's equipment data for the proper cutting
oxygen pressures for the specific cutting top.
The oxidation of iron by this method is highly exothermic. Once started, steel can
be cut at a surprising rate, far faster than if it was merely melted through. At this
point, the pre-heat jets are there purely for assistance. The rise in temperature will
be obvious by the intense glare from the ejected material, even through proper
goggles. (A thermic lance is a tool which also uses rapid oxidation of iron to cut
through almost any material.)
Since the melted metal flows out of the workpiece, there must be room on the
opposite side of the workpiece for the spray to exit. When possible, pieces of
metal are cut on a grate that lets the melted metal fall freely to the ground. The
same equipment can be used for oxyacetylene blowtorches and welding torches,
by exchanging the part of the torch in front of the torch valves.
For a basic oxy-acetylene rig, the cutting speed in light steel section will usually be
nearly twice as fast as a petrol-driven cut-off grinder. The advantages when cutting large
sections are obvious - an oxy-fuel torch is light, small and quiet and needs very little
effort to use, whereas a cut-off grinder is heavy and noisy and needs considerable
operator exertion and may vibrate severely, leading to stiff hands and possible longterm repetitive strain injury. Oxy-acetylene torches can easily cut through ferrous
materials in excess of 50 mm (2 inches). Oxygen Lances are used in scrapping operations
and cut sections thicker than 200 mm (8 inches). Cut-off grinders are useless for these
kinds of application.
Robotic oxy-fuel cutters sometimes use a high-speed divergent nozzle. This uses an
oxygen jet that opens slightly along its passage. This allows the compressed oxygen to
expand as it leaves, forming a high-velocity jet that spreads less than a parallel-bore
nozzle, allowing a cleaner cut. These are not used for cutting by hand since they need
very accurate positioning above the work. Their ability to produce almost any shape from
large steel plates gives them a secure future in shipbuilding and in many other industries.
Oxy-propane torches are usually used for cutting up scrap to save money, as LPG is far
cheaper joule-for-joule than acetylene, although propane does not produce acetylene's
very neat cut profile. Propane also finds a place in production, for cutting very large
sections.
Oxy-acetylene can only cut low to medium carbon steels and wrought iron. High carbon
steels cannot be cut because the melting point is very close to the temperature of the
flame, and so the slag from the cutting action does not eject as sparks, but rather mixes
with the clean melt near the cut. This keeps the oxygen from reaching the clean metal and
burning it. In the case of cast iron, graphite between the grains and the shape of the grains
themselves interfere with cutting action of torch.
Inspection and Testing Methods
A variety of inspection and testing methods are available to check the quality of the
welded joint. Standardized procedures have been developed and specified over the years
FIGURE 1.34 (a) Desired weld profile for single V-groove weld joint. Same joint but
with seveial weld defects: (b) undercut, in which a portion of the base metal part is
melted away; (c) underfill, a depression in the weld below the level of the adjacent base
metal surface; and id) overlap, in which the web metal spills beyond the joint on to the
surface of the base pan but no fusion occurs.
by engineering and trade societies such as the American Welding Society (AWS). For
purposes of discussion, these inspection and testing procedures can be divided into three
categories: (1) visual, (2) nondestructive, and (3) destructive. Visual Inspection Visual
inspection is no doubt the most widely used welding inspection method. Aninspector
visually examines the weldment for (1) conformance to dimensionakspecifications on the
part drawing, (2) warping, and (3) cracks, cavities, incomplete fusion, and other defects
described in the previous section. The welding inspector also determines if additional
tests are warranted, usually in the nondestructive category. The limitation of visual
inspection is that only surface defects are detectable; internal defects cannot be
discovered by visual methods.
Nondestructive Evaluation The nondestructive inspection group includes a variety of
inspection methods that do not damage the specimen being evaluated. Dye-penetrant
and fluorescent-penetrant tests are methods for detecting small defects such as cracks and
cavities that are open to the surface. Fluorescent penetrants are highly visible when
exposed to ultraviolet light. Their use is therefore a more sensitive technique than dyes.
Magnetic particle testing is limited to ferromagnetic materials. A magnetic field is
established in the subject part, and magnetic particles (e.g., iron filings) are sprinkled on
the surface. Subsurface defects such as cracks and inclusions reveal themselves by
distorting the magnett field, causing the particles to be concentrated in certain regions on
the surface. Ultrasonic testing involves the use of high-frequency sound waves (over 20
kHz) directed through the specimen. Discontinuities (e.g., cracks, inclusions, porosity)
are detected by losses in sound transmission. Radiographic testing uses X-rays or
gamma radiation to detect flaws internal to the weld metal. It provides a photographic
film record of any defects.
Destructive Testing: These are methods in which the weld is destroyed either during the
test or to prepare the test specimen. They include mechanical and metallurgical tests.
Mechanical tests are similar in purpose to conventional testing methods such as tensile
tests and shear tests. The difference is that the test specimen is a weld joint. Figure 1.35
presents a sampling of the mechanical tests used in welding. Metallurgical tests involve
the reparation of metallurgical specimens of the weldment to examine such features as
metallic structure, defects, extent and condition of heat-affected zone, presence of other
elements, and similar phenomena.
FIGURE 1.35 Mechanical tests used in welding: (a) tension-shear test of arc weldrnent,
(b) fillet break test, (c) tension-shear test of spot weld, (d) peel test for spot weld.
Oxygen Lance cutting:
The oxygen lance cutting process uses a consumable carbon steel tube. The tip of the tube
is heated to its kindling temperature. A high pressure oxygen flow is started through the
lance. The oxygen reacts with the hot lance tip, releasing sufficient heat to sustain the
reaction.
An oxyfuel torch is usually used to heat the lance tip to a red hot reaction temperature.
Other heat sources include electric resistance and electric arcing. Once the oxygen stream
is started, it rects with the lance material, which results in the creation of both a high
temperature and heat releasing reaction.
The intense reaction of the lance allows it to be used to cut through a variety of materials.
The hot metal leaving the lance tip has not completed its exothermic reaction. As this
reactive mass impacts the surface of the material being cut, it releases a large quantity of
energy in to that surface. Thermal conductivity between the molten metal and the base
material is a very efficient method of heat transfer. This along with the continued burning
of the lance material on the surface, causes the base material to become molten.
Once the base material is molten, it may react with the burning lance material, forming
fumes or slag, which is then blown from the cut. Any molten material not becoming
reactive is carried out of the cut with the slag or blown out with the oxygen stream.
The addition of steel rods or other metals to the center of the oxygen lance tube have
increased their productivity. The improved lances last longer and cut faster.
Schematic view of oxygen lance cutting
Carbon Arc Cutting
Air carbon arc cutting is an arc cutting process in which metals to be cut are melted by
the heat of a carbon arc. The molten metal is removed by a blast of air. This is a method
for cutting or removing metal by melting it with an electric arc and then blowing away
the molten metal with a high velocity jet of compressed air. The air jet is external to the
consumable carbon-graphite electrode. It strikes the molten metal immediately behind the
arc. Air carbon arc cutting and metal removal differ from plasma arc cutting in that they
employ an open (un constricted) arc, which is independent of the gas jet. The air carbon
arc process is shown in figure 1-36.
FIGURE 1.36: Process diagram for air carbon arc cutting
Braze welding:
Braze welding is a procedure used to join two pieces of metal. It is very similar to fusion
welding with the exception that the base metal is not melted. The filler metal is
distributed on to the metal surfaces by tinning. Braze welding often produces bonds that
are comparable to those made by fusion welding without the destruction of the base metal
characteristics. Braze welding is also called bronze welding.
Braze welding has many advantages over fusion welding. It allows you to join dissimilar
metals, to minimize heat distortion, and to reduce extensive pre heating. Another side
effect of braze welding is the elimination of stored up stresses that are often present in
fusion welding. This is extremely important in the repair of large castings. The
disadvantages are the loss of strength when subjected to high temperatures and the
inability to withstand high stresses.
Soldering is a process in which two or more metal items are joined together by melting
and flowing a filler metal into the joint, the filler metal having a relatively low melting
point. Soft soldering is characterized by the melting point of the filler metal, which is
below 400 °C (752 °F).The filler metal used in the process is called solder.
Soldering is distinguished from brazing by use of a lower melting-temperature filler
metal. The filler metals are typically alloys that have liquidus temperatures below 350°C.
It is distinguished from welding by the base metals not being melted during the joining
process which may or may not include the addition of a filler metal. In a soldering
process, heat is applied to the parts to be joined, causing the solder to melt and be drawn
into the joint by capillary action and to bond to the materials to be joined by wetting
action. After the metal cools, the resulting joints are not as strong as the base metal, but
have adequate strength, electrical conductivity, and water-tightness for many uses.
Applications
One of the most frequent applications of soldering is assembling electronic
components to printed circuit boards (PCBs). Another common application is making
permanent but reversible connections between copper pipes in plumbing systems. Joints
in
sheet
metal
objects
such
as
food
cans, roof
flashing, rain
gutters and
automobile radiators have also historically been soldered, and occasionally still
are. Jewelry components are assembled and repaired by soldering. Small mechanical
parts are often soldered as well. Soldering is also used to join lead came and copper
foil in stained glass work. Soldering can also be used as a semi-permanent patch for a
leak in a container or cooking vessel.
One guideline to consider when soldering is that, since soldering temperatures are so low,
a soldered joint has limited service at elevated temperatures. Solders generally do not
have much strength, so the process should not be used for load-bearing members.
Some examples of solder types and their applications include tin-lead (general purpose),
tin-zinc for joining aluminum, lead-silver for strength at higher than room temperature,
cadmium-silver for strength at high temperatures, zinc-aluminium for aluminium and
corrosion resistance, and tin-silver and tin-bismuth for electronics.
Brazing is a metal-joining process whereby a filler metal is heated above and distributed
between two or more close-fitting parts by capillary action. The filler metal is brought
slightly above its melting (liquidus) temperature while protected by a suitable
atmosphere, usually a flux. It then flows over the base metal (known as wetting) and is
then cooled to join the workpieces together. It is similar to soldering, except the
temperatures used to melt the filler metal is above 450 °C (842 °F), or, as traditionally
defined in the United States, above 800 °F (427 °C).
In order to obtain high-quality brazed joints, parts must be closely fitted, and the base
metals must be exceptionally clean and free of oxides. In most cases, joint clearances of
0.03 to 0.08 mm (0.0012 to 0.0031 in) are recommended for the best capillary action and
joint strength. However, in some brazing operations it is not uncommon to have joint
clearances around 0.6 mm (0.024 in). Cleanliness of the brazing surfaces is also of vital
importance, as any contamination can cause poor wetting. The two main methods for
cleaning parts, prior to brazing are chemical cleaning, and abrasive or mechanical
cleaning. In the case of mechanical cleaning, it is of vital importance to maintain the
proper surface roughness as wetting on a rough surface occurs much more readily than on
a smooth surface of the same geometry.
Another consideration that cannot be over-looked is the effect of temperature and time on
the quality of brazed joints. As the temperature of the braze alloy is increased, the
alloying and wetting action of the filler metal increases as well. In general, the brazing
temperature selected must be above the melting point of the filler metal. However, there
are several factors that influence the joint designer's temperature selection. The best
temperature is usually selected so as to: (1) be the lowest possible braze temperature, (2)
minimize any heat effects on the assembly, (3) keep filler metal/base metal interactions to
a minimum, and (4) maximize the life of any fixtures or jigs used. In some cases, a higher
temperature may be selected to allow for other factors in the design (e.g. to allow use of a
different filler metal, or to control metallurgical effects, or to sufficiently remove surface
contamination). The effect of time on the brazed joint primarily affects the extent to
which the aforementioned effects are present; however, in general most production
processes are selected to minimize brazing time and the associated costs. This is not
always the case, however, since in some non-production settings, time and cost are
secondary to other joint attributes (e.g. strength, appearance).
1.15.
Summary
In this unit we have studied Weld Joint, Advantages and Disadvantages of Welded Joints,
Types of Welded Joints, Cold Pressure Welding, Types of Welded Joints, Fillet Welded
Joints, Edge Preparation and Applications, Welding Positions, Black Smith’s Forge
Welding, Electric Resistance Welding, Types of Electric Resistance Welding, Spot
Welding, Roll Spot and Seam Welding, Projection Welding, Butt Welding, Percussion
Welding, Arc Welding, Polarity in Arc Welding, Comparison Between A.C. and D.C.
Arc Welding, Types of Arc Welding, Electrodes for Arc Welding, Arc Welding
Equipment, Precautions in Arc Welding, Arc Welding Processes, Carbon Arc Welding,
Metal Arc Welding, Metallic Inert-gas (MIG)Arc Welding, Tungsten Inert-gas (TIG)Arc
Welding, Atomic Hydrogen Welding, Stud Welding, Submerged Arc Welding, Plasma
Arc Welding, Flux Cored Arc Welding, Electro-slag Welding, Electro-gas Welding,
Thermit Welding, Solid State Welding, Modern Welding Processes, Basic Weld
Symbols, Supplementary Weld Symbols, Elements of a Welding Symbol, Standard
Location of Elements of a Welding Symbol, Gas Welding, Equipment for Oxy-acetylene
Gas Welding, Welding Rods, Fluxes, Gas Flame, Gas Welding Technique, Gas or
Oxygen Cutting of Metals, Cutting Machines, Oxygen Lance Cutting, Arc Cutting,
Oxygen Arc Cutting Process, Welding of Various Metals, Testing of Welded Joints,
Braze Welding, Soldering, Brazing.
1.16.
Keywords
•
Butt joint
•
Resistance welding
•
Comer joint
•
Resistance Spot Welding
•
Lap joint
•
Arc Welding
•
Tee joint
•
Gas Metal Arc Welding
•
Edge joint
•
Electro gas welding
•
Groove welds
•
Laser-Beam Welding
•
Pressure welding
•
Electroslag welding
•
Cold Welding
•
Thermit Welding
•
Forge Welding
•
Oxyfuel Gas Welding
1.17.
Exercise
1. What are the different types of joints?
2. Explain the welding positions.
3. What are the different types of welds? Explain.
4. Write short note on:
a. Butt Welding Process
b. Arc Welding
c. Electroslag Welding
d. Solid-State Welding
e. Oxyfuel Gas Welding
UNIT 2
RECENT DEVELOPMENT IN MANUFACTURING PROCESS
Structure
2.1.
Introduction
2.2.
Objectives
2.3.
Components of Numerical Control
2.4.
NC Part Programming
2.5.
Applications of Numerical Control
2.6.
Advantages & Disadvantages
2.7.
Direct Numerical Control
2.8.
Summary
2.9.
Keywords
2.10.
Exercise
2.1.Introduction
Numerical control (NC) is a form of programmable automation in which the mechanical
actions of a piece of equipment are controlled by a program containing coded
alphanumeric data. The data represent relative positions between a work head and a work
part. The work head is a tool or other processing element, and the work part is the object
being processed. The operating principle of NC is to control the motion of the work head
relative to the work part and to control the sequence in which the motions are carried out.
The first application of numerical control was in machining, and this is still an important
application area.
The Technology of Numerical Control
In this section we define the components of a numerical control system, and then proceed
to describe the coordinate axis system and motion controls.
2.2.Objectives
After studying this unit we are able to understand
− Components of Numerical Control
− NC Part Programming
− Applications of Numerical Control
− Advantages & Disadvantages
− Direct Numerical Control
2.3.Components of Numerical Control
Components of an NC System A numerical control system consists of three basic
components: (1) part program, (2) machine control unit, and (3) processing equipment.
The part program (the term commonly used in machine tool technology) is the detailed
set of commands to be followed by the processing equipment. Each command specifies a
position or motion that is to be accomplished by the workhead relative to the processed
object. A position is defined by its x-y-z coordinates. In machine tool applications,
additional details in the NC program include spindle rotation speed, spindle direction,
feed rate, tool change instructions, and other commands related to the operation. For
many years, NC part programs were encoded on 1-in-wide punched paper tape, using a
standard format that could be interpreted by the machine control unit. Today, punched
tape has largely been replaced by newer storage technologies in modern machine shops.
These technologies include magnetic tape and electronic transfer of NC part programs
from a central computer. The machine control unit (MCU) in modern NC technology is a
microcomputer that stores the program and executes it by converting each command into
actions by the processing equipment, one command at a time. The MCU consists of both
hardware and software. The hardware includes the microcomputer, components to
interface with the processing equipment, and certain feedback control elements. The
MCU may also include a tape reader if the programs are loaded into computer memory
from punched tape. The software in the MCU includes control system software,
calculation algorithms, and translation software to convert the NC part program into a
usable format for the MCU. The MCU also permits the part program to be edited in case
the program contains errors, or changes in cutting conditions are required. Because the
MCU is a computer, the term computer numerical control (CNC) is often used to
distinguish this type of NC from its technological predecessors that were based entirely
on hard-wired electronics. The processing equipment accomplishes the sequence of
processing steps to transform the starting workpart into a completed part. It operates
under the control of the machine control unit according to the set of instructions
contained in the part program.
Coordinate System and Motion Control in NC A standard coordinate axis system is
used to specify positions in numerical control. The system consists of the three linear
axes (x, y, z ) of the Cartesian coordinate system, plus three rotational axes (a, b, c), as
shown in Figure 2.1(a). The rotational axes are used to rotate the workpart to present
different surfaces for machining, or to orient the tool or workhead at some angle relative
to the part. Most NC systems do not require all six axes. The simplest NC systems (e.g.,
plotters, press working machines for flat sheet-metal stock, and component insertion
machines) are positioning systems whose locations can be defined in an x-y plane.
Programming of these machines involves specifying a sequence of x-y coordinates. By
contrast, some machine tools have five-axis control to shape complex workpart
geometries. These systems typically include three linear axes plus two rotational axes.
The coordinates for a rotational NC system are illustrated in Figure 29.2(b). These
systems are associated with turning operations on NC lathes. Although the work rotates,
this is not one of the controlled axes. The cutting path of the lathe tool relative to the
rotating workpiece is defined in the x-z plane, as shown in our figure. In many NC
systems, the relative movements between the processing element and the workpart are
accomplished by fixing the part to a work table and then controlling the positions and
motions of the table relative to a stationary or semistationary workhead.
FIGURE 2.1 Coordinate systems used in nutnericai control: (a) for flat and prismatic
work, and (b) for rotational work
Most machine tools and component insertion machines are based on this method of
operation. In other systems, the workpart is held stationary and the workhead is moved
along two or three axes. Flame cutters, x-y plotters, and coordinate measuring machines
operate in this mode.
Motion control systems based on NC can be divided into two types: (1) point-to point
and (2) continuous path. Point-to-point systems, also called positioning systems, move
the workhead (or workpiece) to a programmed location with no regard for the path taken
to get Lo that location. Once the move is completed, some processing action is
accomplished by the workhead at the location, such as drilling or punching a hole. Thus,
the program consists of a series of point locations at which operations are performed.
Continuous path systems provide continuous simultaneous control of more than one axis,
thus controlling the path followed by the tool relative to the part. This permits the tool to
perform a process while the axes are moving, enabling the system to generate angular
surfaces, two-dimensional curves, or three-dimensional contours in the workpart. This
operating scheme is required in drafting machines, certain milling and turning operations,
and flame cutting. In machining, continuous path control also goes by the name
contouring.
Another aspect of rnotion control is concerned with whether the positions in the
coordinate system are defined absolutely or incrementally. In absolute positioning, the
workhead locations are always defined with respect to the origin of the axis system. In
incremental posilioning, the next workhead position is defined relative to the present
location. The difference is illustrated in Figure 2.2.
FIGURE 2.2 Absolute vs. incremental positioning. The workhead is at point (2,3) and is
to be moved to point (6,8). In absolute positioning, the move is specified by x = 6, y = 8;
while in incremental positioning, the move is specified by x=4, y =5.
2.4.NC Part Programming
In machine tool applications, the task of programming the system is called NC part
programming because the program is prepared for agiven part. It is usually accomplished
by someone familiar with the metalworking process who has learned the programming
procedure for the particular equipment in the plant. For other processes, other terms may
be used for programming, but the principles are similar and a trained individual is needed
to prepare the program. Computer systems are used extensively to prepare NC programs.
Part programming requires the programmer to define the points, lines, and surfaces of the
workpart in the axis system, and to control the movement of the cutting tool relative to
these defined part features. Several part programming techniques are available, the most
important of which are (1) manual part programming, (2) computer-assisted part
programming, (3) CADICAM-assisted part programming, and (4) manual data input.
Manual Part Programming For simple point-to-point machining jobs, such as drilling
operations, manual programming is often the easiest and most economical method.
Manual part programming uses basic numerical data and special alphanumeric codes to
define the steps in the process. For example, to perform a drilling operation, a command
of the following type is entered:
Each4'word" in the statement specifies a detail in the drilling operation. The n-word
(n010) is simply a sequence number for the statement. The x- and y-words indicate the x
and y coordinate positions (x =70.0 mm and y = 85.5 mm). The f-word and s-word
specify the feed rate and spindle speed to be used in the drilling operation (feed rate =
175 m/min and spindle speed = 500 rev/min). The complete NC part program consists of
a sequence of statements similar to the above command. Computer-Assisted Part
Programming Computer-assisted part programming involves the use of a high-level
programming language. It is suited to the programming of more complex jobs than
manual programming. The first part programming language was APT (Automatically
Programmed Tooling), developed as an extension of the original NC machine tool
research and first used in production around 1960.
In APT, the part programming task is divided into two steps: (1) definition of part
geometry and (2) specification of tool path and operation sequence. In step 1, the part
programmer defines the geometry of the workpart by means of basic geometric elements
such as points, lines, planes, circles, and cylinders. These elements are defined using APT
geometry statements, such as
P1 is a point defined in the*-y plane located at x = 25 mm and y = 150 mm. L1 is a line
that goes through points PI and P2. Similar statements can be used to define circles,
cylinders, and other geometry elements. Most workpart shapes can be described using
statements like these to define their surfaces, corners, edges, and hole locations.
Specification of the tool path is accomplished with APT motion statements. A typical
statement for point-to-point operation is
GOTO/Pl
This directs the tool to move from its current location to a position defined by PI, where
PI has been defined by a previous APT geometry statement. Continuous path motion
commands use geometry elements such as lines, circles, and planes. For example,
consider the command
GORGT/L3, PAST, L4
The statement directs the tool to go right (GORGT) along line L3 until it is positioned
just past line L4 (of course, L4 must be a line that intersects L3).
Additional APT statements are used to define operating parameters such as feed rates,
spindle speeds, tool sizes, and tolerances. When completed, the part programmer enters
the APT program into the computer, where it is processed to generate low-level
statements (similar to statements prepared in manual part programming) that can be used
by a particular machine tool.
CADICAM-Assisted Part Programming: The use of CADICAM takes computer assisted
part programming a step further by using a computer graphics system (CADICAM
system) to interact with the programmer as the part program is being prepared. In the
conventional use of APT, a complete program is written and then entered into the
computer for processing. Many programming errors are not detected until computer
processing. When a CADICAM system is used, the programmer receives immediate
visual verification when each statement is entered, to determine whether the statement is
correct. When part geometry is entered by the programmer, the element is graphically
displayed on the monitor. When the tool path is constructed, the programmer can see
exactly how the motion commands will move the tool relative to the part. Errors can be
corrected immediately rather than after the entire program has been written. Interaction
between programmer and programming system is a significant benefit of CADICAMassisted programming. There are other important benefits of using CADICAM in NC part
programming. First, the design of the product and its components may have been
accomplished on a CADICAM system. The resulting design database, including the
geometric definition of each part, can be retrieved by the NC programme to use as the
starting geometry for part programming. This retrieval saves valuable time compared to
reconstructing the part from scratch using the APT geometry statements.
Second, special software routines are available in CADICAM-assisted part programming
to automate portions of the tool path generation, such as profile milling around the
outside periphery of a part, milling a pocket into the surface of a part, surface contouring,
and certain point-to-point operations. These routines are called by the part programmer as
special macro commands. Their use results in significant savings in programming time
and effort.
Manual Data Input: Manual data input (MDI) is a method in which a machine operator
enters the part program in the factory. The method involves use of a CRT display with
graphics capability at the machine tool controls. NC part programming statements are
entered using a menu-driven procedure that requires minimum training of the machine
tool operator. Because part programming is simplified and does not require a special staff
of NC part programmers, MDI is a way for small machine shops to economically
implement numerical control into their operations.
2.5.Applications of Numerical Control
Machining is an important application area for numerical control, but the operating
principle of CNC can be applied to other operations as well. There are many industrial
processes in which the position of a workhead must be controlled relative to the part or
product being worked on. We divide the applications into two categories: (1) machine
tool applications, and (2) non machine tool applications. It should be noted that the
applications are not all identified by the name numerical control in their respective
industries. In the machine tool category, NC is widely used for machining operations
such as turning, drilling, and milling. The use of NC in these processes has motivated the
development of highly automated machine tools called machining centers, which change
their own cutting tools to perform a variety of machining operations under NC program
control . In addition to machining, other numerically controlled machine tools include (1)
grinding machines; (2) sheet metal press working machines; (3) tube-bending machines ;
and (4) thermal cutting processes.
In the non machine tool category, NC applications include (1) tape-laying machines and
filament-winding machines for composites; (2) welding, machines, both arc welding and
resistance welding; (3) component-insertion machines in electronics assembly; (4)
drafting machines; and (5) coordinate measuring machines for inspection.
Benefits of NC relative to manually operated equipment in these applications include (1)
reduced nonproductive time, which results in shorter cycle times, (2) lower
manufacturing lead limes, (3) simpler fixturing, (4) greater manufacturing flexibility, (5)
improved accuracy, and (6) reduced human error.
2.6.Advantages & Disadvantages of NC
Advantages of NC:
1. CNC machines can be used continuously 24 hours a day, 365 days a year and only
need to be switched off for occasional maintenance.
2. CNC machines are programmed with a design which can then be manufactured
hundreds or even thousands of times. Each manufactured product will be exactly the
same.
3. Less skilled/trained people can operate CNCs unlike manual lathes / milling machines
etc.. which need skilled engineers.
4. CNC machines can be updated by improving the software used to drive the machines
5. Training in the use of CNCs is available through the use of ‘virtual software’. This is
software that allows the operator to practice using the CNC machine on the screen of a
computer. The software is similar to a computer game.
6. CNC machines can be programmed by advanced design software such as
Pro/DESKTOP®, enabling the manufacture of products that cannot be made by manual
machines, even those used by skilled designers / engineers.
7. Modern design software allows the designer to simulate the manufacture of his/her
idea. There is no need to make a prototype or a model. This saves time and money.
8. One person can supervise many CNC machines as once they are programmed they can
usually be left to work by themselves. Sometimes only the cutting tools need replacing
occasionally.
9. A skilled engineer can make the same component many times. However, if each
component is carefully studied, each one will vary slightly. A CNC machine will
manufacture each component as an exact match.
Disadvantages of NC:
1. CNC machines are more expensive than manually operated machines, although costs
are slowly coming down.
2. The CNC machine operator only needs basic training and skills, enough to supervise
several machines. In years gone by, engineers needed years of training to operate centre
lathes, milling machines and other manually operated machines. This means many of the
old skills are been lost.
3. Less workers are required to operate CNC machines compared to manually operated
machines. Investment in CNC machines can lead to unemployment.
4. Many countries no longer teach pupils / students how to use manually operated lathes /
milling machines etc... Pupils / students no longer develop the detailed skills required by
engineers of the past. These include mathematical and engineering skills.
2.7.Direct numerical control
Direct numerical control (DNC), also known as distributed numerical control (also
DNC), is a common manufacturing term for networking CNC machine tools. On some
CNC machine controllers, the available memory is too small to contain the machining
program (for example machining complex surfaces), so in this case the program is stored
in a separate computer and sent directly to the machine, one block at a time. If the
computer is connected to a number of machines it can distribute programs to different
machines as required. Usually, the manufacturer of the control provides suitable DNC
software. However, if this provision is not possible, some software companies provide
DNC applications that fulfill the purpose. DNC networking or DNC communication is
always required when CAM programs are to run on some CNC machine control.
2.8.Summary
In this unit we have studied Working of NC Machines tools, Components of NC
Machines, Programming for NC Machines, Methods of Listing the Co-ordinates of points
in NC System, Application of NC Machine, Advantages & Disadvantages, Direct
Numerical Control.
2.9.Keywords
Numerical control
Manual Part Programming
Manual data input
2.10.
Exercise
1. What are the different components of numerical control?
2. Explain the applications of numerical control.
3. What are advantages and disadvantages of numerical control?
Unit 3
DRILLING MACHINES
Structure
3.1.
Introduction
3.2.
Objectives
3.3.
Construction of Drilling Machine
3.4.
Types of Drilling Machine
3.5.
Types of Drills
3.6.
Operations Performed on Drilling Machine
3.7.
Size of a Drilling Machine
3.8.
Summary
3.9.
Keywords
3.10.
Exercise
3.1.
Introduction
Drilling is an operation of making a circular hole by removing a volume of metal from the
job by cutting tool called drill. A drill is a rotary end-cutting tool with one or more cutting
lips and usually one or more flutes for the passage of chips and the admission of cutting fluid.
A drilling machine is a machine tool designed for drilling holes in metals. It is one of the
most important and versatile machine tools in a workshop. Besides drilling round holes,
many other operations can also be performed on the drilling machine such as counter- boring,
countersinking, honing, reaming, lapping, sanding etc.
3.2.
Objectives
After studying this unit we are able to understand
− Construction of Drilling Machine
− Types of Drilling Machine
− Types of Drills
− Operations Performed on Drilling Machine
− Size of a Drilling Machine
3.3.
Construction of Drilling Machine
In drilling machine the drill is rotated and fed along its axis of rotation in the stationary
workpiece. Different parts of a drilling machine are shown in Fig. 3.1 and are discussed
below: (i) The head containing electric motor, V-pulleys and V-belt which transmit rotary
motion to the drill spindle at a number of speeds. (ii) Spindle is made up of alloy steel. It
rotates as well as moves up and down in a sleeve. A pinion engages a rack fixed onto the
sleeve to provide vertical up and down motion of the spindle and hence the drill so that the
same can be fed into the workpiece or withdrawn from it while drilling. Spindle speed or the
drill speed is changed with the help of V-belt and V-step-pulleys. Larger drilling machines
are having gear boxes for the said purpose. (iii) Drill chuck is held at the end of the drill
spindle and in turn it holds the drill bit. (iv) Adjustable work piece table is supported on the
column of the drilling machine. It can be moved both vertically and horizontally. Tables are
generally having slots so that the vise or the workpiece can be securely held on it. (v) Base
table is a heavy casting and it supports the drill press structure. The base supports the column,
which in turn, supports the table, head etc. (vi) Column is a vertical round or box section
which rests on the base and supports the head and the table. The round column may have rack
teeth cuton it so that the table can be raised or lowered depending upon the workpiece
requirements. This machine consists of following parts
1. Base
2. Pillar
3. Main drive
4. Drill spindle
5. Feed handle
6. Work table
Fig. 3.1 Construction of drilling machine
3.4.
Types of Drilling Machine
Drilling machines are classified on the basis of their constructional features, or the type of
work they can handle. The various types of drilling machines are:
(1) Portable drilling machine
(2) Sensitive drilling machine
(a) Bench mounting
(b) Floor mounting
(3) Upright drilling machine
(a) Round column section
(b) Box column section machine
(4) Radial drilling machine
(a) Plain
(b) Semiuniversal
(c) Universal
(5) Gang drilling machine
(6) Multiple spindle drilling machine
(7) Automatic drilling machine
(8) Deep hole drilling machine
(a) Vertical
(b) Horizontal
Few commonly used drilling machines are described as under.
3.4.1 Portable Drilling Machine
A portable drilling machine is a small compact unit and used for drilling holes in worpieces
in any position, which cannot be drilled in a standard drilling machine. It may be used for
drilling small diameter holes in large castings or weldments at that place itself where they are
lying. Portable drilling machines are fitted with small electric motors, which may be driven
by both A.C. and D.C. power supply. These drilling machines operate at fairly high speeds
and accommodate drills up to 12 mm in diameter.
3.4.2 Sensitive Drilling Machine
It is a small machine used for drilling small holes in light jobs. In this drilling machine, the
workpiece is mounted on the table and drill is fed into the work by purely hand control. High
rotating speed of the drill and hand feed are the major features of sensitive drilling machine.
As the operator senses the drilling action in the workpiece, at any instant, it is called sensitive
drilling machine. A sensitive drilling machine consists of a horizontal table, a vertical
column, a head supporting the motor and driving mechanism, and a vertical spindle. Drills of
diameter from 1.5 to 15.5 mm can be rotated in the spindle of sensitive drilling machine.
Depending on the mounting of base of the machine, it may be classified into following types:
1. Bench mounted drilling machine, and
2. Floor mounted drilling machine
3.4.3 Upright Drilling Machine
The upright drilling machine is larger and heavier than a sensitive drilling machine. It is
designed for handling medium sized workpieces and is supplied with power feed
arrangement. In this machine a large number of spindle speeds and feeds may be available for
drilling different types of work. Upright drilling machines are available in various sizes and
with various drilling capacities (ranging up to 75 mm diameter drills). The table of the
machine also has different types of adjustments. Based on the construction, there are two
general types of upright drilling machine:
(1) Round column section or pillar drilling machine.
(2) Box column section.
The round column section upright drilling machine consists of a round column whereas the
upright drilling machine has box column section. The other constructional features of both
are same. Box column machines possess more machine strength and rigidity as compared to
those having round section column.
3.4.4 Radial Drilling Machine
Fig. 3.2 illustrates a radial drilling machine. The radial drilling machine consists of a heavy,
round vertical column supporting a horizontal arm that carries the drill head. Arm can be
raised or lowered on the column and can also be swung around to any position over the work
and can be locked in any position. The drill head containing mechanism for rotating and
feeding the drill is mounted on a radial arm and can be moved horizontally on the guide-ways
and clamped at any desired position. These adjustments of arm and drilling head permit the
operator to locate the drill quickly over any point on the work. The table of radial drilling
machine may also be rotated through 360 deg. The maximum size of hole that the machine
can drill is not more than 50 mm. Powerful drive motors are geared directly into the head of
the machine and a wide range of power feeds are available as well as sensitive and geared
manual feeds. The radial drilling machine is used primarily for drilling medium to large and
heavy workpieces. Depending on the different movements of horizontal arm, table and drill
head, the upright drilling machine may be classified into following types1. Plain radial drilling machine
2. Semi universal drilling machine, and
3. Universal drilling machine.
Fig. 3.2 Radial drilling machine
In a plain radial drilling machine, provisions are made for following three movements 1. Vertical movement of the arm on the column,
2. Horizontal movement of the drill head along the arm, and
3. Circular movement of the arm in horizontal plane about the vertical column.
In a semi universal drilling machine, in addition to the above three movements, the drill head
can be swung about a horizontal axis perpendicular to the arm. In universal machine, an
additional rotatory movement of the arm holding the drill head on a horizontal axis is also
provided for enabling it to drill on a job at any angle.
3.4.5 Gang Drilling Machine
In gang drilling machine, a number of single spindle drilling machine columns are placed
side by side on a common base and have a common worktable. A series of operation may be
performed on the job by shifting the work from one position to the other on the worktable.
This type of machine is mainly used for production work.
3.4.6 Multiple-Spindle Drilling Machine
The multiple-spindle drilling machine is used to drill a number of holes in a job
simultaneously and to reproduce the same pattern of holes in a number of identical pieces in a
mass production work. This machine has several spindles and all the spindles holding drills
are fed into the work simultaneously. Feeding motion is usually obtained by raising the
worktable.
3.5.
Types of Drills
A drill is a multi point cutting tool used to produce or enlarge a hole in the workpiece. It
usually consists of two cutting edges set an angle with the axis. Broadly there are three types
of drills:
1. Flat drill,
2. Straight-fluted drill, and
3. Twist drill
Flat drill is usually made from a piece of round steel which is forged to shape and ground to
size, then hardened and tempered. The cutting angle is usually 90 deg. and the relief or
clearance at the cutting edge is 3 to 8 deg. The disadvantage of this type of drill is that each
time the drill is ground the diameter is reduced. Twist drill is the most common type of drill
in use today. The various types of twist drills (parallel shank type and Morse taper shank
type) are shown in Fig. 3.3
Fig. 3.3 Types of twist drill
Number sizes
In metric system, the drill is generally manufactured from 0.2 to 100 mm. In British system
the drills sizes range from No. 1 to No. 80. Number 80 is the smallest having diameter equal
to 0.0135 inch and the number 1 is the largest having diameter equal to 0.228 inch. Number 1
to number 60 is the standard sets of drills. The numbers 61 to 80 sizes drills are not so
commonly used. The diameter of drills increases in steps of approximately by 0.002 inch.
Letter sizes
The drill sizes range from A to Z, A being the smallest having diameter equal to 0.234 inch
and Z being the largest having diameter equal to 0.413 inch, increasing in steps of
approximately O.010 inch fractional sizes: The drill sizes range from 1/64" inch to 5 inch in
steps of 1/64 inches up to 1.75 inches, then the steps gradually increase. The drill sizes range
from A to Z, A being the smallest having diameter equal to 0.234 inch and Z being the largest
having diameter equal to 0.413 inch, increasing in steps of approximately O.010 inch
fractional sizes: The drill sizes range from 1/64" inch to 5 inch in steps of 1/64 inches up to
1.75 inches, then the steps gradually increase. The drill is generally removed by tapping a
wedge shaped drift into the slot in the drilling machine spindle as shown in Fig. 3.4.
Fig. 3.4 Removing a drill from drill machine
3.5.1 Twist Drill Geometry
Twist drill geometry and its nomenclature are shown in Fig. 3.5. A twist drill has three
principal parts:
(i) Drill point or dead center
(ii) Body
(iii) Shank.
Drill axis is the longitudinal centre line.
Drill point is the sharpened end of the drill body consisting of all that part which is shaped to
produce lips, faces and chisel edge.
Lip or cutting edge is the edge formed by the intersection of the flank and face
Lip length is the minimum distance between the outer corner and the chisel-edge corner of
the lip.
Face is that portion of the flute surface adjacent to the lip on which the chip impinges as it is
cut from the work.
Chisel edge is the edge formed by the intersection of the flanks.
Flank is that surface on a drill point which extends behind the lip to the following flute.
Flutes are the grooves in the body of the drill, which provide lips, allow the removal of chips,
and permit cutting fluid to reach the lips.
Flute length is the axial length from the extreme end of the point to the termination of the
flutes at the shank end of the body.
Body is that portion of the drill nomenclature, which extends from the extreme cutting end to
the beginning of the shank.
Shank is that portion of the drill by which it is held and driven,
Heel is the edge formed by the intersection of the flute surface and the body clearance.
Body clearance is that portion of the body surface reduced in diameter to provide diametric
clearance.
Core or web is the central portion of the drill situated between the roots of the flutes and
extending from the point end towards the shank; the point end of the core forms the chisel
edge.
Lands are the cylindrically ground surfaces on the leading edges of the drill flutes. The width
of the land is measured at right angles to the flute.
Recess is the portion of the drill body between the flutes and the shank provided so as to
facilitate the grinding of the body. Parallel shank drills of small diameter are not usually
provided with a recess.
Outer corner is the corner formed by the intersection of the lip and the leading edge of the
land.
Chisel edge comer is the corner formed by the intersection of a lip and the chisel edge.
Drill diameter is the measurement across the cylindrical lands at the outer corners of the
drill. .
Lead of helix is the distance measured parallel to the drill axis between corresponding points
on the leading edge of a flute in one complete turn of the flute.
Helix angle is the angle between the leading edge of the land and the drill axis.
Rake angle is the angle between the face and a line parallel to the drill axis. It is bigger at the
face edges and decreases towards the center of the drill to nearly 0°. The result is that the
formation of chips grows more un-favorable towards the centre.
Lip clearance angle is the angle formed by the flank and a plane at right angles to the drill
axis; the angle is normally measured at the periphery of the drill. To make sure that the main
cutting edges can enter into the material, the clearance faces slope backwards in a curve. The
clearance angle is measured at the face edge, must amount to 5° up to 8°.
Point angle is the included angle of the cone formed by the lips.
Fig. 3.5 Geometry and nomenclature of twist drill
3.5.2 Drill Material
Drills are made are made up of high speed steel. High speed steel is used for about 90 per
cent of all twist drills. For metals more difficult to cut, HSS alloys of high cobalt series are
used.
3.6.
Operations Performed on Drilling Machine
A drill machine is versatile machine tool. A number of operations can be performed on it.
Some of the operations that can be performed on drilling machines are:
1. Drilling
2. Reaming
3. Boring
4. Counter boring
5. Countersinking
6. Spot facing
7. Tapping
8. Lapping
9. Grinding
10. Trepanning.
The operations that are commonly performed on drilling machines are drilling, reaming,
lapping, boring, counter-boring, counter-sinking, spot facing, and tapping. These operations
are discussed as under.
3.6.1 Drilling
This is the operation of making a circular hole by removing a volume of metal from the job
by a rotating cutting tool called drill as shown in Fig. 3.6. Drilling removes solid metal from
the job to produce a circular hole. Before drilling, the hole is located by drawing two lines at
right angle and a center punch is used to make an indentation for the drill point at the center
to help the drill in getting started. A suitable drill is held in the drill machine and the drill
machine is adjusted to operate at the correct cutting speed. The drill machine is started and
the drill starts rotating. Cutting fluid is made to flow liberally and the cut is started. The
rotating drill is made to feed into the job. The hole, depending upon its length, may be drilled
in one or more steps. After the drilling operation is complete, the drill is removed from the
hole and the power is turned off.
Fig. 3.6 Drilling operation
3.6.2 Reaming
This is the operation of sizing and finishing a hole already made by a drill. Reaming is
performed by means of a cutting tool called reamer as shown in Fig. 3.7. Reaming operation
serves to make the hole smooth, straight and accurate in diameter. Reaming operation is
performed by means of a multitooth tool called reamer. Reamer possesses several cutting
edges on outer periphery and may be classified as solid reamer and adjustable reamer.
Fig. 3.7 Reaming operation
3.6.3 Boring
Fig. 3.8 shows the boring operation where enlarging a hole by means of adjustable cutting
tools with only one cutting edge is accomplished. A boring tool is employed for this purpose.
Fig. 3.8 Boring operation
3.6.4 Counter-Boring
Counter boring operation is shown in Fig. 3.9. It is the operation of enlarging the end of a
hole cylindrically, as for the recess for a counter-sunk rivet. The tool used is known as
counter-bore.
Fig. 3.9 Counter boring operation
3.6.5 Counter-Sinking
Counter-sinking operation is shown in Fig. 3.10. This is the operation of making a
coneshaped enlargement of the end of a hole, as for the recess for a flat head screw. This is
done for providing a seat for counter sunk heads of the screws so that the latter may flush
with the main surface of the work.
Fig. 3.10 Counter sinking operation
3.6.6 Lapping
This is the operation of sizing and finishing a hole by removing very small amounts of
material by means of an abrasive. The abrasive material is kept in contact with the sides of a
hole that is to be lapped, by the use of a lapping tool.
3.6.7 Spot-Facing
This is the operation of removing enough material to provide a flat surface around a hole to
accommodate the head of a bolt or a nut. A spot-facing tool is very nearly similar to the
counter-bore
3.6.8 Tapping
It is the operation of cutting internal threads by using a tool called a tap. A tap is similar to a
bolt with accurate threads cut on it. To perform the tapping operation, a tap is screwed into
the hole by hand or by machine. The tap removes metal and cuts internal threads, which will
fit into external threads of the same size. For all materials except cast iron, a little lubricate
oil is applied to improve the action. The tap is not turned continuously, but after every half
turn, it should be
Fig. 3.11 Tapping operation
reversed slightly to clear the threads. Tapping operation is shown in Fig.3.11. The geometry
and nomenclature of a tap is given in Fig. 3.12.
Fig. 3.12 Geometry and nomenclature of tap
3.6.9 Core drilling
Core drilling operation is shown in Fig. 3.13. It is a main operation, which is performed on
radial drilling machine for producing a circular hole, which is deep in the solid metal by
means of revolving tool called drill.
Fig. 3.13 Core drilling operation
3.7.
Size of a Drilling Machine
Different parameters are being considered for different types of drilling machines to
determine their size. The size of a portable drilling machine is decided by the maximum
diameter of the drill that it can hold. The sensitive and upright drilling machines are specified
by the diameter of the largest workpiece which can be centered under the drill machine
spindle. A radial drilling machine is specified by the length of the arm and the diameter of the
column. To specify a drilling machine completely, following other parameters may also be
needed:
1. Table diameter
2. Number of spindle speeds and feeds available
3. Maximum spindle travel
4. Morse taper number of the drill spindle
5. Power input
6. Net weight of the machine
7. Floor space required, etc.
•
Cutting Speed
The cutting speed in a drilling operation refers to the peripheral speed of a point on the
surface of the drill in contact with the work. It is usually expressed in meters/min. The cutting
speed (Cs) may be calculated as:
Cs = ((22/7) × D × N)/1000
Where, D is the diameter of the drill in mm and
N is the rpm of the drill spindle.
•
Feed
The feed of a drill is the distance the drill moves into the job at each revolution of the spindle.
It is expressed in millimeter. The feed may also be expressed as feed per minute. The feed per
minute may be defined as the axial distance moved by the drill into the work per minute. The
feed per minute may be calculated as:
F = Fr × N
Where, F = Feed per minute in mm.
Fr = Feed per revolution in mm.
N = R.P.M. of the drill.
3.8.
Summary
In this unit we have studied
− Construction of Drilling Machine
− Types of Drilling Machine
− Types of Drills
− Operations Performed on Drilling Machine
− Size of a Drilling Machine
3.9.
Keywords
Portable drilling machine
Sensitive drilling machine
Bench mounting
Floor mounting
Upright drilling machine
Round column section
Box column section machine
Radial drilling machine
Semiuniversal
Universal
Gang drilling machine
Multiple spindle drilling machine
Automatic drilling machine
Deep hole drilling machine
Flat drill,
Straight-fluted drill, and
Twist drill
3.10.
Exercise
1. State the working principle of a drilling machine.
2. Explain principal parts of the drilling machine and sketch the mechanism of a drilling
machine.
3. Give the classification of drilling machines.
4. How will you specify a drilling machine?
5. What operations can be done on a drilling machine? Discuss them with diagrams.
6. With the help of a line diagram, describe the construction of radial drilling machine.
7. List the devices commonly used for holding the work on a drilling machine, and describe
any three.
8. Define cutting speed, feed and machining time for drilling.
9. Sketch a twist drill and name its different parts.
10. What is boring? Sketch a boring tool.
11. What is the function of flutes on a twist drill bit? Why are straight flute drills used for
nonferrous materials and metal?
12. Draw suitable figure for a drill bit showing:
(i) point (ii) lip clearance (iii) point angle (iv) flute (v) margin and (vi) body clearance
13 Write short notes on following:
(i) Drilling (ii) Boring, (iii) Reaming (iv) Tapping (v) Counter boring (vi) Counter sinking
14. Explain various types of operations performed on a drilling machine by neat sketches.
15. Define the following terms used in drilling operation.
(i) Cutting speed (ii) Feed
Unit 4
SHAPER, PLANNER AND SLOTTING MACHINE
Structure
4.1.
Introduction
4.2.
Objectives
4.3.
Working Principle of Shaper
4.4.
Types of Shapers
4.4.1 Crank Shaper
4.4.2 Geared Shaper
4.4.3 Hydraulic Shaper
4.4.4 Standard Shaper
4.4.5 Universal Shaper
4.4.6 Horizontal Shaper
4.4.7 Vertical Shaper
4.4.8 Travelling Head Shaper
4.4.9 Push Type Shaper
4.4.10 Draw Type Shaper
4.5.
Principal Parts of Shaper
4.6.
Specification of a Shaper
4.7.
Shaper Mechanism
4.7.1 Crank and Slotted Link Mechanism
4.8.
Planer
4.9.
Working Principal of Planer
4.10.
Difference between Shaper and Planer
4.11.
Types of Planers
4.12.
Slotter
4.13.
Principle Parts of a Slotter
4.14.
Operations Performed on a Slotting Machine
4.15.
Summary
4.16.
Keywords
4.17.
Exercise
4.1.
Introduction
Shaper is a reciprocating type of machine tool in which the ram moves the cutting tool
backwards and forwards in a straight line. The basic components of shaper are shown in Fig.
4.1. It is intended primarily to produce flat surfaces. These surfaces may be horizontal,
vertical, or inclined. In general, the shaper can produce any surface composed of straight-line
elements. The principal of shaping operation is shown in Fig. 4.2 (a, b). Modern shapers can
also generate contoured surface as shown in Fig. 4.3. A shaper is used to generate flat (plane)
surfaces by means of a single point cutting tool similar to a lathe tool.
Fig. 4.1 Principal components of a shaper
4.2.
Objectives
After studying this unit we are able to understand
− Working Principle of Shaper
− Types of Shapers
− Principal Parts of Shaper
− Specification of a Shaper
− Shaper Mechanism
− Planer
− Working Principal of Planer
− Difference between Shaper and Planer
− Types of Planers
− Slotter
− Principle Parts of a Slotter
− Operations Performed on a Slotting Machine
4.3.
Working Principle of Shaper
A single point cutting tool is held in the tool holder, which is mounted on the ram. The
workpiece is rigidly held in a vice or clamped directly on the table. The table may be
supported at the outer end. The ram reciprocates and thus cutting tool held in tool holder
moves forward and backward over the workpiece. In a standard shaper, cutting of material
takes place during the forward stroke of the ram. The backward stroke remains idle and no
cutting takes place during this stroke. The feed is given to the workpiece and depth of cut is
adjusted by moving the tool downward towards the workpiece. The time taken during the idle
stroke is less as compared to forward cutting stroke and this is obtained by quick return
mechanism. The cutting action and functioning of clapper box is shown in Fig.4.4 during
forward and return stroke.
Fig. 4.2 (a, b) Working principal of shaping machine
Fig. 4.3 Job surfaces generated by shaper
Fig. 4.4 Cutting action and functioning of clapper box
4.4.
Types of Shapers
Shapers are classified under the following headings:
(1) According to the type of mechanism used for giving reciprocating motion to the ram
(a) Crank type
(b) Geared type
(c) Hydraulic type
(2) According to the type of design of the table:
(a) Standard shaper
(b) Universal shaper
(3) According to the position and travel of ram:
(a) Horizontal type
(b) Vertical type
(c) Traveling head type
(4) According to the type of cutting stroke:
(a) Push type
(b) Draw type.
A brief description these shapers is given below4.4.1 Crank Shaper
This is the most common type of shaper. It employs a crank mechanism to change circular
motion of a large gear called “bull gear” incorporated in the machine to reciprocating motion
of the ram. The bull gear receives power either from an individual motor or from an overhead
line shaft if it is a belt-driven shaper.
4.4.2 Geared Shaper
Geared shaper uses rack and pinion arrangement to obtain reciprocating motion of the ram.
Presently this type of shaper is not very widely used.
4.4.3 Hydraulic Shaper
In hydraulic shaper, reciprocating motion of the ram is obtained by hydraulic power. For
generation of hydraulic power, oil under high pressure is pumped into the operating cylinder
fitted with piston. The piston end is connected to the ram through piston rod. The high
pressure oil causes the piston to reciprocate and this reciprocating motion is transferred to the
ram of shaper. The important advantage of this type of shaper is that the cutting speed and
force of the ram drive are constant from the very beginning to the end of the cut.
4.4.4 Standard Shaper
In standard shaper, the table has only two movements, horizontal and vertical, to give the
feed.
4.4.5 Universal Shaper
A universal shaper is mostly used in tool room work. In this type of shaper, in addition to the
horizontal and vertical movements, the table can be swiveled about an axis parallel to
perpendicular to the first axis.
4.4.6 Horizontal Shaper
In this type of shaper, the ram holding the tool reciprocates in a horizontal axis.
4.4.7 Vertical Shaper
In vertical shaper, the ram reciprocates in a vertical axis. These shapers are mainly used for
machining keyways, slots or grooves, and internal surfaces.
4.4.8 Travelling Head Shaper
In this type of shaper, the ram while it reciprocates, also moves crosswise to give the required
feed.
4.4.9 Push Type Shaper
This is the most general type of shaper used in common practice, in which the metal is
removed when the ram moves away from the column, i.e. pushes the work.
4.4.10 Draw Type Shaper
In this type of shaper, the cutting of metal takes place when the ram moves towards the
column of the machine, i.e. draws the work towards the machine. The tool is set in a reversed
direction to that of a standard shaper.
4.5.
Principal Parts of Shaper
Fig. 4.5 shows the parts of a standard shaper. The main parts are given as under.
1. Base
2. Column
3. Cross-rail
4. Saddle
5. Table
6. Ram
7. Tool head
8. Clapper box
9. Apron clamping bolt
10. Down feed hand wheel
11. Swivel base degree graduations
12. Position of stroke adjustment hand wheel
13. Ram block locking handle
14. Driving pulley
15. Feed disc
16. Pawl mechanism
17. Elevating screw
Some of important parts are discussed as under.
Fig. 4.5 Parts of a standard shaper
•
Base
It is rigid and heavy cast iron body to resist vibration and takes up high compressive load. It
supports all other parts of the machine, which are mounted over it. The base may be rigidly
bolted to the floor of the shop or on the bench according to the size of the machine.
•
Column
The column is a box shaped casting mounted upon the base. It houses the ram-driving
mechanism. Two accurately machined guide ways are provided on the top of the column on
which the ram reciprocates.
•
Cross rail
Cross rail of shaper has two parallel guide ways on its top in the vertical plane that is
perpendicular to the rai1 axis. It is mounted on the front vertical guide ways of the column.
It consists mechanism for raising and lowering the table to accommodate different sizes of
jobs by rotating an elevating screw which causes the cross rail to slide up and down on the
vertical face of the column. A horizontal cross feed screw is fitted within the cross rail and
parallel to the top guide ways of the cross rail. This screw actuates the table to move in a
crosswise direction.
•
Saddle
The saddle is located on the cross rail and holds the table on its top. Crosswise movement of
the saddle by rotation the cross feed screw by hand or power causes the table to move
sideways.
•
Table
The table is a box like casting having T -slots both on the top and sides for clamping the
work. It is bolted to the saddle and receives crosswise and vertical movements from the
saddle and cross rail.
•
Ram
It is the reciprocating part of the shaper, which reciprocates on the guideways provided above
the column. Ram is connected to the reciprocating mechanism contained within the column.
•
Tool head
The tool head of a shaper performs the following functions(1) It holds the tool rigidly,
(2) It provides vertical and angular feed movement of the tool, and
(3) It allows the tool to have an automatic relief during its return stroke.
The various parts of tool head of shaper are apron clamping bolt, clapper box, tool post, down
feed, screw micrometer dial, down feed screw, vertical slide, apron washer, apron swivel pin,
and swivel base. By rotating the down feed screw handle, the vertical slide carrying the tool
gives down feed or angular feed movement while machining vertical or angular surface. The
amount of feed or depth of cut may be adjusted by a micrometer dial on the top of the down
feed screw. Apron consisting of clapper box, clapper block and tool post is clamped upon the
vertical slide by a screw. The two vertical walls on the apron called clapper box houses the
clapper block, which is connected to it by means of a hinge pin. The tool post is mounted
upon the clapper block. On the forward cutting stroke the clapper block fits securely to the
clapper box to make a rigid tool support. On the return stroke a slight frictional drag of the
tool on the work lifts the block out of the clapper box a sufficient amount preventing the tool
cutting edge from dragging and consequent wear. The work surface is also prevented from
any damage due to dragging.
4.6.
Specification of a Shaper
The size of a shaper is specified by the maximum length of stroke or cut it can make. Usually
the size of shaper ranges from 175 to 900 mm. Besides the length of stroke, other particulars,
such as the type of drive (belt drive or individual motor drive), floor space required, weight of
the machine, cutting to return stroke ratio, number and amount of feed, power input etc. are
also sometimes required for complete specification of a shaper.
4.7.
Shaper Mechanism
In a shaper, rotary motion of the drive is converted into reciprocating motion of the ram by
the mechanism housed within the column or the machine. In a standard shaper metal is
removed in the forward cutting stroke, while the return stroke goes idle and no metal is
removed during this period as shown in Fig. 4.4. The shaper mechanism is so designed that it
moves the ram holding the tool at a comparatively slower speed during forward cutting
stroke, whereas during the return stroke it allow the ram to move at a faster speed to reduce
the idle return time. This mechanism is known as quick return mechanism. The reciprocating
movement of the ram and the quick return mechanism of the machine are generally obtained
by anyone of the following methods:
(1) Crank and slotted link mechanism
(2) Whitworth quick return mechanism, and
(2) Hydraulic shaper mechanism
The crank and slotted link mechanism is discussed as under.
4.7.1 Crank and Slotted Link Mechanism
In crank and slotted link mechanism (Fig. 4.6), the pinion receives its motion from an
individual motor or overhead line shaft and transmits the motion or power to the bull gear.
Bull gear is a large gear mounted within the column. Speed of the bull gear may be changed
by different combination of gearing or by simply shifting the belt on the step cone pulley. A
radial slide is bolted to the centre of the bull gear. This radial slide carries a sliding block into
which the crank pin is fitted. Rotation of the bull gear will cause the bush pin to revolve at a
uniform speed. Sliding block, which is mounted upon the crank pin is fitted within the slotted
link. This slotted link is also known as the rocker arm. It is pivoted at its bottom end attached
to the frame of the column. The upper end of the rocker arm is forked and connected to the
ram block by a pin. With the rotation of bull gear, crank pin will rotate on the crank pin
circle, and simultaneously move up and down the slot in the slotted link giving it a rocking
movement, which is communicated to the ram. Thus the rotary motion of the bull gear is
converted to reciprocating motion of the ram.
Fig. 4.6 Crank and slotted link mechanism
•
Surfaces Produced on Shaper
1. Horizontal plain surface
2. Vertical plain surface
3. Inclined surface
4. Grooved surface
5. Slotted surface
6. Stepped surface
•
Shaper Operations
A shaper is a machine tool primarily designed to generate a flat surface by a single point
cutting tool. Besides this, it may also be used to perform many other operations. The different
operations, which a shaper can perform, are as follows:
1. Machining horizontal surface (Fig. 4.7)
2. Machining vertical surface (Fig. 4.8)
3. Machining angular surface (Fig. 4.9)
4. Slot cutting (Fig. 4.10)
5. Key ways cutting (Fig. 4.11)
6. Machining irregular surface (Fig. 4.12)
7. Machining splines and cutting gears (Fig. 4.13)
4.8.
Planer
Like a shaper, planer is used primarily to produce horizontal, vertical or inclined flat surfaces
by a single point cutting tool. But it is used for machining large and heavy workpieces that
cannot be accommodated on the table of a shaper. In addition to machining large work, the
planer is frequently used to machine multiple small parts held in line on the platen. Planer is
mainly of two kinds namely open housing planer and double housing planer. The principle
parts of the open housing planer are shown in Fig 4.14(a). The principle parts of the double
housing planer are shown in Fig 4.14(b). The bigger job is fixed with help of the grooves on
the base of the planer and is accurately guided as it travels back and forth. Cutting tools are
held in tool heads of double housing planer and the work piece is clamped onto the worktable
as shown in Fig. 4.14(b). The worktable rides on the gin tool heads that can travel from side
to side i.e., in a direction at right angle to the direction of motion of the worktable. Tool heads
are mounted on a horizontal cross rail that can be moved up and down. Cutting is achieved by
applying the linear primary motion to the workpiece (motion X) and feeding the tool at right
angles to this motion (motion Y and Z). The primary motion of the worktable is normally
accomplished by a rack and pinion drive using a variable speed motor. As with the shaper,
the tool posts are mounted on clapper boxes to prevent interference between the tools and
work-piece on the return stroke and the feed motion is intermittent. The size of a standard
planer is specified by the size of the largest solid that can reciprocate under the tool. In
addition to this, some other parameters such as table size (length and width), type of drive,
number of speeds and feeds available, power input, weight of the machine, floor space
required etc. may be required to specify a planer completely.
Fig. 4.14 Principle parts of double housing planer
4.9.
Working Principal of Planer
Fig. 4.15 depicts the working principle of a planer. In a planer, the work which is supported
on the table reciprocates past the stationary cutting tool and the feed is imparted by the lateral
movement of the tool. The tool is clamped in the tool holder and work on the table. Like
shaper, the planner is equipped with clapper box to raise the tool in idle stroke. The different
mechanisms used to give reciprocating motion to the table are following1. Reversible motor drive
2. Open and cross belt drive
3. Hydraulic drive
4.10.
Difference between Shaper and Planer
The difference between shaper and planner is given in Table 4.1.
Table 4.1 Difference between Shaper and Planer
4.11.
Types of Planers
Planers may be classified in a number of ways, but according to general construction,
these are the following types:
1. Double housing planer
2. Open side planer
3. Pit planer
4. Edge or plate type planer
5. Divided table planer
4.12.
Slotter
The slotter or slotting machine is also a reciprocating type of machine tool similar to a shaper
or a planer. It may be considered as a vertical shaper. The chief difference between a shaper
and a slotter is the direction of the cutting action. The machine operates in a manner similar
to the shaper, however, the tool moves vertically rather than in a horizontal direction. The job
is held stationary. The slotter has a vertical ram and a hand or power operated rotary table.
4.13.
Principle Parts of a Slotter
Fig. 4.16 shows a slotter and its various parts. The main parts of a slotter are discussed as
under:
•
Bed or Base
It is made up of cast iron. It supports column, tables, ram, driving mechanism etc. The top of
the bed carries horizontal ways along which the worktable can traverse.
•
Table
It holds the work piece and is adjustable in longitudinal and cross-wise directions. The table
can be rotated about its centre.
•
Hand wheels
They are provided for rotating the table and for longitudinal and cross traverse.
•
Column is the vertical member
They are made up of cast iron and it houses the driving mechanism. The vertical front face of
the column is accurately finished for providing ways along which the ram moves up and
down.
•
Ram
It is provided to reciprocate vertically up and down. At its bottom, it carries the cutting tool.
It is similar to the ram of a shaper; but it is more massive and moves vertically, at right angle
to the worktable, instead of having the horizontal motion of a shaper:
•
Cross-slide
It can be moved parallel to the face of the column. The circular work-table is mounted on the
top of the cross-slide.
Fig. 4.16 Slotter and its various parts
4.14.
Operations Performed on a Slotting Machine
A slotter is a very economical machine tool when used for certain classes of work given as
under.
(i) It is used for machining vertical surfaces
(ii) It is used angular or inclined surfaces
(iii) It is used It is used to cut slots, splines keyways for both internal and external jobs
such as machining internal and external gears,
(iv) It is used for works as machining concave, circular, semi-circular and convex surfaces
(v) It is used for shaping internal and external forms or profiles
(vi) It is used for machining of shapes which are difficult to produce on shaper
(vii) It is used for internal machining of blind holes
(viii) It is used for machining dies and punches, and
Since a slotter works slowly. It has less use in mass production work. It can be substituted by
the broaching machine.
4.15.
Summary
In this unit we have studied
− Working Principle of Shaper
− Types of Shapers
− Principal Parts of Shaper
− Specification of a Shaper
− Shaper Mechanism
− Planer
− Working Principal of Planer
− Difference between Shaper and Planer
− Types of Planers
− Slotter
− Principle Parts of a Slotter
− Operations Performed on a Slotting Machine
4.16.
Keywords
Crank Shaper
Geared Shaper
Hydraulic Shaper
Standard Shaper
Universal Shaper
Horizontal Shaper
Vertical Shaper
Travelling Head Shaper
Push Type Shaper
Draw Type Shaper
Double housing planer
Open side planer
Pit planer
Edge or plate type planer
Divided table planer
4.17.
Exercise
1. Explain principal parts of a shaper by neat sketch.
2. How can you classify the shapers?
3. How can you specify a shaper?
4. Explain the principle of quick return motion mechanism of a shaper. What is need of this
mechanism?
5. Using neat sketches, describe the various operations that can be carried on shaping
machines.
6. Explain various safety precautions associated with shaper.
7. Explain principal parts of a planer by neat sketches.
8. State the working principle of a planer.
9. Using neat sketch show the various parts of a planer.
10. Classify planers? Explain the universal planer.
11. Show by neat sketches various types of planer tools.
12. How table reversal is obtained in a planer?
13. Using neat sketches, describe the various operations that can be carried on planer.
14. Make neat sketch of a slotter. Explain its working with applications.
15. Differentiate between shaper, planer and slotter.